A few ounces? Treat it with dry gas. More than a few ounces? Drop the tank and clean it out.
The only way to know for sure if a person has water in the gas tank is to drain the tank. Usually a car will sputter and die if there is water in the tank.
remove the fuel tank and drain the water
What will happen if water gets inside gas tank
skipping stall no start
No they will not. You don't 'treat' a tank you treat its water. There are additives that you can put in the water to remove chlorine if you are using chlorinated tap/reticulated water. Failure to use one of these additives could result in damage to the fishes gills and protective slime layer. Chlorine is a gas and will usually evaporate off after a few hours particularly in a tropical tank with a filter running.
bubbles on top of gas in tank
A small amount, say a couple ounces of water you could probably treat with a good dry gas treatment. Any more than that would require removal of the fuel tank, then removal of the fuel sending unit/fuel pump assembly from the tank, then draining, cleaning and drying could be effectively done.
Water is heavier than gasoline, it will pool at the lowest point of the fuel tank.
Have you tried using gas line antifreeze? That will help to remove some of the water in the gas. The best way to make certain all the water is out of the tank is to siphon the gas out, let the tank air out and re-fill the tank with gas from a station you know does not have water in the gas.
Yes, water is heavier than gas.
Remove the tank from the car.