water → purified water → hydrogen/oxygen → burn hydrogen
produces around double the amount of energy it consumes. – Even without efficiency modules.

I assume you miscalculated the fuel values, when you "nerved" hydrogen.
I would recommend reducing the fuel value of hydrogen and remove the ability to add efficiency modules to electrolysers.

This is not just a obscurity but for seablock the by far cheapest way to produce energy. Even cheaper than solar power.
And additionally it works on nights and only early tech is required.

See:
https://youtu.be/_smLtG865mI

hmmmm... It's possible, but unlikely, that something changed after I did the calculations, but I was fairly sure, that with the setup you're using there, you shouldn't be able to get back as much energy as it took to make the Hydrogen.

Also, be aware I'm not actually modding anymore, I'll be handing my mods off to somebody else when I find the right person, or people, so if a change is required here, it could be a while before it happens.

but I was fairly sure, that with the setup you're using there, you shouldn't be able to get back as much energy as it took to make the Hydrogen.
You can see the whole world on that video. So it /is/ possible. – Even without efficency-modules.
I found that out by scripting the calculations to see what I am loosing if I throw away hydrogen and reproduce it. So it is no "bug" but the real values.

I was looking over your source code but I didn't find out how you calculated the values. Schould one hydrogen be one mol of hydrogen or one gram or one atom? And did you consider efficiency modules? So the amount to produce it should 5 times the amount chemical energy needed so that the 20% efficiency cap makes 100% efficiency?

I could provide patches. If you are interested. But as you see on my activity not very fast.

but I was fairly sure, that with the setup you're using there, you shouldn't be able to get back as much energy as it took to make the Hydrogen.

You can see the whole world on that video. So it /is/ possible. – Even without efficency-modules.
I found that out by scripting the calculations to see what I am loosing if I throw away hydrogen and reproduce it elsewhere. So it is no "bug" but the real values.

I was looking over your source code but I didn't find out how you calculated the values. Schould one hydrogen be one mol of hydrogen or one gram or one atom? And did you consider efficiency modules? So the amount to produce it should 5 times the amount chemical energy needed so that the 20% efficiency cap makes 100% efficiency?

I could provide patches. If you are interested. But as you see on my activity not very fast.

as far as power is concerned, 10 units is 500 gram.

the calculations are a bit strange too, I think 1 item is 1 kilogram, with a fuel value of half the real world value (due to the game halving all fuel values after I made this determination), which fits for coal.
then 10 units of a fluid is equivalent to 1 item.

Hydrogen on the other hand was not calculated using this method, it was calculated from the amount of hydrogen it costs to make other fuels (or how much you get from petroleum gas), and the recipes were tweaked at the same time to be similar to this value across the board.
so, basically, the value of hydrogen is as it currently is, because it's tied to the fuel value of fuel blocks, petroleum gas, and Hydrazine.

Hydrazine however had it's value tweaked up when the halving happened, as efficiency multipliers were removed from the generators at the same time, and I Wanted more of a reason to use that fuel specifically.

So... the way everything is tied together, you can't just tweak the value of Hydrogen and still have it make sense, especially since the hydrogen fuel value is already way lower than it's real world value. (When later making Deuterium, heavy hydrogen, I used the real world calculated value of Hydrogen for Deuterium)

Part of the solution was to change the electrolyser's running cost, multiplying it up to make hydrogen more costly to make, pushing the energy return value to below the cost.

There were plans to add a multiplier to make them even more expensive... hang on a minute. (Loads game and checks)

In Bobplates (MCI (Metals, Chemicals and Intermediates)) Turn on Expensive Electrolysis mode, it's turned off by default, that will fix your Hydrogen energy return issue.
I completely forgot about this setting until describing things just now.

This is not just a obscurity but for seablock the by far cheapest way to produce energy. Even cheaper than solar power.

This isn't possible in a Sea Block game as it makes additional adjustments.

@KiwiHawk: I made it in a seablock game. But I had a lot of additional mods. So I don't know how the interaction was there. I can retry in a plain seablock.
@Bobingabout: I assumed such connections. I will look into it.

especially since the hydrogen fuel value is already way lower than it's real world value
And I assume this is (in parts) the problem: If I'm right you used the fuel value of hydrogen to calculate the price of certain rezipes. So if you are using other methods it go to the same target it fails since where using real values there.
I assume this cant be fixed without changing the recipes of fuel blocks.
as efficiency multipliers were removed from the generators
One problem is that they are not removed from electrolysers.

@KiwiHawk: I made it in a seablock game. But I had a lot of additional mods. So I don't know how the interaction was there. I can retry in a plain seablock.

If you can reproduce it in a Sea Block game, please post your mod list on the Sea Block discord, GitHub, or mod page and I'll fix it.

https://discord.gg/veAEAbGSaB
https://github.com/KiwiHawk/SeaBlock/issues
https://mods.factorio.com/mod/SeaBlock

OK it's a little bit more complicated.

as far as power is concerned, 10 units is 500 gram.

If I assume that petrolium is methane (like the icon) or petrol/Gasoline (which would make sense from the recipes as it can cracked from light oil). And liquid fuel is Diesel fuel it makes sense for these two.
I assume you lowered the fuel values of heavy/light/crude oil by purpose that it is more efficient to split crude oil and to make liquid fuel.

then 10 units of a fluid is equivalent to 1 item.

But for coal its wrong: 500g of coal should give ~15MJ per 500g. It gives only 4.

so, basically, the value of hydrogen is as it currently is, because it's tied to the fuel value of fuel blocks, petroleum gas, and Hydrazine.

I really don't get that chain: To get Hydrazine i need bobrevamp but than the solid fuel recipes are deactivated (but still there).
If I use bobrevamp and assume that H₂ should have the energy of the petroleum that is used to produce it (it should be much higher), 46kJ would be right. And than it fits to hydrazine.
But this does not match to the solid fuel values. 175H₂ have 8Mj. But the fuel value is 12MJ.
For making it from petroleum you would have 20*2.3MJ=46MJ. Much more expensive.

Turn on Expensive Electrolysis mode
Yes. This fixes the problem for electrolyzer 1. When you add bobassembly it breaks since electrolyter 2 consumes only 1.5 times the energy by doubeling the speed. Together with 2 efficiency modules the cycle will be net positive again.

Was fairly sure I calculated it based on Electrolyser 5... so you can't get power from an unmodded electrolyser at all.
however, modules would still break past even.

Yes. With expensive electrolysers 5 without modules I will loose energy. (I was just assuming, that you calculated with electrolyser 1 and 80% energy-loss because it fitted exactly.) You can make energy by producing enriched fuel from coal or deuterium from Water. But I assume, this is intended, since there is no way back. Still I would like to know how you calculated the fuel value of coal.

Yes. With expensive electrolysers 5 without modules I will loose energy. (I was just assuming, that you calculated with electrolyser 1 and 80% energy-loss because it fitted exactly.) You can make energy by producing enriched fuel from coal or deuterium from Water. But I assume, this is intended, since there is no way back. Still I would like to know how you calculated the fuel value of coal.

I honestly don't remember how I factored the fuel value of coal into it. I obviously didn't come up with it myself, as that's a base game item, and I didn't modify it.
Coal to Fuel blocks gives you more energy out than the items that you put in would, because it gives you incentive to actually do it.
Deuterium might have a high fuel value, but considering it's rarity, you're unlikely to want to burn it anyway. I mean, you could, but you have to deal with all the pure water too.