gap filler for bathroom that is waterproof?

[marknoo]

Is there a gap filler used for noise, heat insulation that is also waterproof? I am thinking of gap filler like the "Great Stuff" brand type.

I'm trying to keep the floor under my shower solid/intact until I can junk this trailer.
and there has been a leak in the tube, it seems likely it has been there for quite some time. The floor still feels solid but . . .