I'm a swimmer not a triathlete (trashed knee, never could run anyhow). These days I bike more than swim. The triathlon is biased so swimming takes the shortest % time of the 3 legs (which is another topic), so for a beginner I believe the objective would be to finish the swim reasonably comfortably so you are in good shape to hammer and pound away on the bike/run.
Swimming is a technique-limited sport. Getting some feedback and knowlegeable coaching will likely improve your time and required effort much more than just pounding out yardage at the pool. Other than The Speedo (ranked right up there with The Bike Shorts), swimming is a VERY Clyde-friendly activity. Body fat == buoyancy; you won't go faster but it is less fatiguing which is helpful for beginners.
Flexibility is also something to work on, primarily in ankles and shoulders. In the pool, you can save a lot of energy with a good streamline off the walls. Breathing is also crucial to swimming comfortably. Exhale deeply and the inhale will take care of itself. Being comfortable in the water means that you relax the muscles that aren't needed for propulsion. When I was a youngster learning how to swim, on the day I learned how to relax and breathe correctly I progressed from struggling through 25 yards to being able to swim virtually as far as I wanted.
My suggestion is to see if you can find a local Masters team to swim with AND either get some books/DVDs or take a weekend swim clinic targeted for adults.