Wow... some of these women on here....
If you can't trust your guy in a pool alone with another woman... then your relationship has issues...
Call it a pool.... call it a carwash... call it a chatty waitress at the diner.... if you can't handle your guy being around other women.. AT ALL, even in a platonic situation... then find a guy thats reserved to not do those things. (you won't find too many that will never talk to another woman again, sorry ladies).
I would have no problems swimming with just another friend.. be it male or female.. and as long as the person I'm with knows about it, and knows the person, and is secure in our relationship, then theres no harm.