Are you already a strong swimmer?
Mostly you'll want to work on technique. If your tri includes an open-water swim make sure you include open-water swimming in your training if you are unfamiliar. If your swim will be a wetsuit swim, swim in your suit and practice taking that baby off quickly.
You need to add some bricks (at least once per week) to your routine. After your bike ride, toss on your running shoes and go for a short run (2-3 miles). You can practice your transitions during this training as well.
Good luck and have fun!