If there is water in the housing it will cause a short. That will cause the breaker to trip.

So long as the flood bulb fits, is the right voltage and wattage I don't see why a regular bulb wouldn't work.

You need to know which hayward light you have. There are many hayward lights, and a variety of lens gaskets. Looking up the light information will also likely tell you what wattage bulb your light is rated for.

Here's some hayward gaskets: http://www.amazon.com/s/ref=nb_sb_ss...rd+lens+gasket