>>96375361
If they can make an Atlas with the same density as a person, or whale (which would be capable of swimming), then it isn't hard to imagine that these uber armour strength styrofoam weight vehicles wouldn't have those same issues.

The materials science is Battletech is crazy advanced but they got poo brained computers that weigh many tons.

My head cannon is just that the mechs are very small and everyone is a miniature, so they don't weigh very much.