I think that depends on how strong you want your upper body to be.
I would think that to gain the strength to swim better/faster, you should do a sport-specific workout in the gym, and, of course, swim lengths.
If it was me, I'd find somebody who was an avid swimmer (maybe somebody who teaches stroke improvement courses) and ask them what they feel are good weight training exercises to increase your strength for swimming. You might even think about enrolling in their course. Work hard. When they see you're interested and taking it seriously, they'll be more willing to answer all your questions
