yes!! the density of the water-methane emulsion is given by:
density= density of water*(1-p) + density of methane (more or less zero) *p
where p=volume fraction of gas.. ... so let us assume that p=0.50 (ie half of the mixture is methane and half water)

then the density of the mixture would be about ONE HALF the density of water.... and so the buoyancy of the ship would be much less because the ship is displacing low density fluid. hence the ship will sink lower in the water...perhaps enough to become submerged totally.