Make ocean water a subtype of /turf/simulated/floor/water

[Atermonera]

Brief description of the issue

And while you're at it, make the depth a var on the turf, not a subtype that other subtypes have to subtype around (See /turf/simulated/floor/water/pool and /turf/simulated/water/deep/pool for why this is a terrible pattern)

This will require touching map files.