Do Snails float when they are dead like fish?

Yes. Just like we would if we died in water
Their corpses become bloated with decomposition gases and this causes them to float, much like a balloon would. And because muscle mass is much denser in a fish's back, this tends to turn the fish upside down.
If they are on their side, yes. It is unusual for a fish to just float to the top of the water and stay there for awhile. It is their instinct to keep swimming around more towards the bottom of the water, unless food is thrown onto the top of the water.