The vehicle  won't start at all, because fuel is lighter than water
and comes up so only water will go to carburator.
The fuel tank should be emptied and completely dried.It maybe effect
the operation of carburator so carburator needs to be cleaned too.

Cars don't run on water, because water doesn't burn.  How bad your
problem really is depends on what a "large" amount is, but you may
find the best thing to do is just to drain the tank and refill it with
known-good gas.

If it's a "small" amount, adding some dry gas will make some of that
water miscible with the gasoline.  But it takes a lot of dry gas to
dissolve a little bit of water.

If you CAN make your car run with water in the gas tank, it should be
good at least for an award from the EPA, if not a Nobel.
--scott