**Update: **I will be uploading the “ships” update sometime tomorrow (or Sunday if something unexpected happens :p), there are still some network jittering problems but there is no point in delaying the update any further.
I will also update the wiki (from ) when I have time with detailed information about buoyancy settings, how the per-poly collision ship is done etc.
I laughed way too hard at 
Have you checked the Min/Z configs in the fish manager?
You can add a thin box collision to the mesh then set the collision and trace channels appropriately so that it only block trace but does not actually collide with anything.
But I don’t think that would be the best method, you could easily get the world location Z of the BP_Ocean.