Hi,
i do spray tans and i'm also a leisure centre manager, in the summer and the rool up to crimbo i spray some of the staff and they go swimming as norm after they've been sprayed. it's not the chlorine in the water that strips the tan (it's only 1.5 - 2.0 ppm, so it's not going to bleach it) it's being in water for a long time, so softening the skin then rubbing yourself dry with a towel that'll strip it off. i always tell the girls to go swimming like normal but don't go too mad with the showering ie don't go scrubbing yourself all over in the shower to get rid of the smell then just wander about with your towel round you till you're dry or gently pat dry and smother yourself in body cream to stop the skin drying out. obviously if they go every day then the tan will fade much quicker same as if they lie in the bath for an hour every night, but if it's only a once a week swim it doesn't do much to the tan. in fact some of my staff will have a spray if they're going to do lifeguard training at another centre so they don't look pasty!!
HTH
Cat x