Its normal for them to soak for periods of time, usually if the cage is to dry and they are about to shed. My BP never soaks even when the cage is to dry she dosnt like the water no idea why.

Sometimes BP will soak for extended period of times if they have mites. So take a look at you BP in the pits and under the head, look for black things wedged in the scales.

Other then that I wouldnt worry about it to much unless the water is really cold. My roomates Red tail used to soak for long periods of time in the water and it would get really cold, so he put a heat lamp over the water too keep it at least room temp.