While most triathlon swims are in open water (a lake or an ocean, and some even in a river), unless you are fortunate enough to live in a handful of places, most of your swim training is likely to be done in a pool. There are plenty of differences between a pool and open water, but pools are sufficient for a good swimming base; and there are drills that can be done in a pool to assist with open water training. For example sighting drills can be done in a pool where you practice looking ahead of you while still going forward. You need to do this in a race since there are not nice lines drawn on the bottom, and it is not an easy skill. Most training plans include specifics about what to do for your swim training days including teaching some tri-specific drills.
Master’s swimming is another way to train that has become very popular among triathletes. This is like swim team for adults. The advantage is the instruction and a more competitive atmosphere to push yourself. Many find swimming very boring, and showing up with a group of friends can really help the boredom, not to mention assisting you in just getting to the pool. But, one downside is that it will not be triathlon specific, nor specific; Master’s programs incorporate all of the different strokes. [Master’s has become so popular among triathletes that there may be some clubs that are triathlon specific. I live in a small town and it hasn’t happened, but I wouldn’t be surprised if some towns do and you could check in your area].
My cynical side thinks that the reason Master’s is recommended by coaches and triathlon literature has to do with the partnership between USAT (USA Triathlon) and USMS (US Master’s Swimming). Regardless, I did Master’s for a few years, learned a lot, and had fun swimming! I would love to be doing it now but it doesn’t fit well for me right at the moment.