I have completed the major refactoring that will allow new boats and ships to be added by other mods! I need some additional testers to try it before posting it on the mod portal. Please give these a try and let me know if they work or not in this thread.

Location of the mod version with the API: https://github.com/robot256/cargo_ships/releases/tag/api-beta-19
Location of my example Electric Boat mod: https://github.com/robot256/electric-boat/releases/tag/beta-1

Just seen the post and have never beta tested anything but am running the mod on a somewhat big base.
What should I be looking out for if anything?

Thanks for taking a look! Main things are building, blueprinting, mining, and destroying of ships and boats. Make sure there are no Lua errors and no invisible ship engines left on the track.

Okay, I can run them for a couple of hours and play a bit destructively to see if something stops working.
Anything more than that goes beyond my abilities, but let's see how it goes.

As a bit of a side note, what niche is the electric ship intended for?
As it looks right now it's less power/acceleration and less compact than nuclear fuel. I suppose reusable though!

Demanding anything would be a bit much, but personally I would be more interested in a nuclear(like the locomotive needing a reactor while building and running on cells as the power plant does) for a faster, greater cost cargo ship.
Though having to refuel underway because some of my shipping lines get really long is an interesting challenge...
The biggest issue, kind of, with using the smaller, faster boat for automated passenger transport is that the bulk and oil freighter cap network speed, so I typically end up grabbing a remote control hover craft with the two mods.

Anyway, I agree that the cargo ships should be substantially slower than trains and have a work around already so it's fine.

Thanks for the feedback. I didn't really try to balance the electric boat yet, but realistically it would have a shorter range than the others and need to stop frequently to swap batteries. Maybe one of the automatic fuel stop mods would let it take advantage of solar recharging outposts along long routes automatically.

Waterways are free, so if I intend to use boats and ships together I always give them parallel or passing tracks.

Once the API is published, other folks can rebalance it, add artillery ships, nuclear ships, whatever they desire.

I was too lazy to build a whole secondary network up to now...
Used remote controlled hovercrafts like a spidertron, but currently boarding one with the spidertron(mod) doesn't work, so more of a hassle.

Okay, that makes a lot of sense regarding the electric test!
It makes sense that they have less range, and it comes with the advantage of reusable fuel cells so that is nice. Still, kind of wished that they had greater acceleration to give them an advantage in exchange for range anxiety.
Looking forward to if anyone does craft some more ship types once you're done!

Anyway, ran 1.19 for some ten or so hours and didn't encounter any issues with blue printing or mining things in a big expansion. Ran smoothly with the one caveat that destroyed engines(train sprite) stick around until the piece of track is rebuilt. Just mining it is not enough.
Something I never noticed before because the ships hardly ever get destroyed.

What do you mean with invisible engines left on the track?

Spent less time testing the electric ones because there was a compatibility issue between the battery and fluid train fuel mods.
Going to have to run it for some longer to see if there could be anything there but will do later as my current save doesn't have the infrastructure and would need some rebuilding which is a hassle right now.

Thanks for the report! Do you have information on how to reproduce the persistent "train sprite"? What entity was destroyed and what was still there?

When you mouse over the ship, you will be able to select two different entities: the "ship" with cargo slots, and the "engine" with fuel slots. The engine is actually a separate entity with blank sprites that is created so it is underneath the sprite of the ship body (cargo wagon) sprite. The mod is supposed to delete any ship engine that is not connected to a ship body, but sometimes it might not. Then you will be able to mouse over a seemingly empty spot of waterway and select a ship engine entity.

Always happens when I blow up a cargo or tanker ship, though now that I tried it with a boat I get it as well.
More noticeable with the cargo or tanker one though because as you said it is made of an underlying engine and wagon combination. So the broken engine parts always appear where the stern/second entity was.(https://imgur.com/UUBofjE)

As you can hopefully see it is just the debris field that happens when one loses an ordinary locomotive. Only that unlike with them I checked that these don't disappear until the ship track has been torn down and rebuild or a new ship placed(not driven) on top of it as if it was a ghost of a destroyed entity.

I never encountered an engine remaining on the tracks as you described. Which is good considering there are somewhere north of 200 cargo ships hauling ore around right now in the save?

Oh! It's the "ruins" associated with the locomotive entity, if it gets destroyed rather than deleted. It will be easy to set the engine ruin to empty. Good catch! (should there be a more realistic shipwrech? Hmm).

I mean, ships do sink beneath the surface. Unlike train wrecks they wouldn't lie around.
You could strew about some flotsam, maybe an oil slick for that Factorio pollution as part of the debris field so typical for when everything that wasn't bolted down floats up after the ship goes down. Not a high priority though.

so far from my testing for compatibility it seems like k2 just overwrites it making it the same as a normal boat again which is not ideal I'm gonna look in to what k2 does and see if it just a flag that needs to be expressed to k2 or a choice the author choosing probably out of simplicity of execution

The main mod with the ships API has been released to the mod portal! The testing-only Electric Boat mod is still available at the GitHub link. I would be happy if someone else adopts it to improve the graphics and balancing.