Boats are a clusterfuck that uses an enum (and ordinals) everywhere to identify the boat type. Even extending the vanilla BoatEntity class would be difficult, as the enum and/or ordinals are also used everywhere that needs the boat type.

Terraform adds boats, but it's extremely hacky.
There really needs to be a way to do this - at this point I won't be adding boats to my mod simply due to there being no viable way for me to do so - but should it go in fapi? I'm not sure.

One third-party solution is to use Terraform's API to add boat types.

I think a real fix would be to add a way to add to enums with fabric's mixin fork, or something similar.

That would break the case when e.g. the client has a mod that adds boats but is connected to a server without that mod, or when the client and the server has mods loaded in different orders (if a certain order isn't guaranteed when adding to enums).