fix issue where ships could get incorrect pathmap blocking zones

[lostsquirrel1]

… with underwater structures near coast lines. Well, anything that floats in the water, not just ships.

[sprunk]

I think isSubmersible was meant for units that can fully submerge, i.e. can go low enough that their top is underwater. But things that float on water will never do that:

RecoilEngine/rts/Sim/MoveTypes/MoveDefHandler.h

Line 186 in bcd9533

// can this unit completely submerge in water?

The meaning may have changed along the way so this may still be the correct fix despite that (I can't really tell) but in that case it would be good to adjust that comment, or perhaps rename the var for the new meaning (blockedBySeafloorObjects or whatever).