Waterlogged blocks with code?

Does anyone know of a way to waterlog blocks, preferably permanently (like kelp)? I've tried messing around with the Vanilla code (as in adding bits of it to my block, not changing it), but it would always create an error when I tried to launch the client.

I'm basically adding a new type of kelp, that is pretty much identical in all ways except appearance, if that makes a difference.