At the pool I swim at I see the adult classes that have people in them from about 35-80, and I see them everyday during the year, it is amazing how quickly they learn, in only 6 months they are swimming, and these are not "athletic" types. On my tri team we have 3 people who swim poorly, but as soon as they get out of the water and on to the bike they pass everyone, so...it certainly won't hold you back in a tri. I swim well, but bike and run slow, and all the slow swimmers pass me within 30 minutes.