Hi ya! Love the mod and came up with an idea on how you could go about resource gathering outside of oil. This might be out of the scope of what you intend only with your mod, but since you’re the only one doing extensive sea-based things, I thought I would at least put this forward. Please see things in all angles and possible implementations, take it apart and see what can be used. So...
Dredges!
That's the answer i came up with, but in a special kind of way.
I notice in factorio, there are different depths of water (This is prob just graphical from my understanding), so i was thinking, if you could have a ship that could dredge the bottom, and make the water deeper, while filling its inventory with "dredgings" (I have no idea what the official name for mud and other detritus dug from the sea bottom is).
These dredgings can then be sent off to be washed and refined, giving a % chance to gain ores (coal should be higher maybe because it’s a fossil fuel?), stone and small amounts of wood (sunken drift wood) and dead fish. The washed off mud can be turned into landfill. (I see the ratio as something like 2% Uranium, 10% iron/copper, 18% fish/wood, 15% stone and the rest washed mud (or make that a base resource that you get regardless.))
The mechanic of making water deeper could also extend to digging out new water channels!
You could also make landfill granular, in the sense, that when using 1 unit, it will increase the layer of depth of water. For example; If you have shallow water, 1 land fill will turn it into land. If you have medium depth water, 1 unit of landfill turns it into shallow water... etc etc...
Now, the only thing about all this is that there needs to be a way to stop exploitation. Like re-digging a spot that's already been filled once. So if you could track what’s been dug already, and what’s not. One way I thought is making a separate set of identical sea/landfill tile sets, that recognise as not mine-able, but that might not work? (Inversely you could make new separate set as only mine-able, so only certain areas of the map can be mine? might make for interesting play)
As for how the dredging might work? There are many ways that it could be implemented. One-way I thought is having an offshore platform like your oil rig, but act as a sort of robo port, when there dredges where like mining robots digging up nearby tiles. I don't know if its possible, but the dredges themselves could, if not placed in a port, could act as a vehicle you can drive around, and need flammables to run. When acting as part of the port, the port refuels then by placing its store of flammables inside it, while its being emptied of dredgings. This way you could have an automated and manual dredging process.
As for mod inter-compatibility, I know angels, as an offshore pump that dose something like this, so it would be just a case of exacting the same item? There are also mods like Dirty mining, and any other mod that adds an ore.
Anyhow, these are just my garbled ramblings, that i at least hope can draw some inspiration from, and if implemented mean you won’t need to go so far as implementing so hard to see under sea ore or something.
(As an aside note or p.s.) you could do what deep core mining did, and create at world gen, ore spots in the sea that act like oil wells, and plonk a deep-sea drill on top? Personally, I like variety, so seeing both this and dredging would be good.
