Whether or not to buy a wetsuit is a personal decision. If you are not sure
you
may want to check local bike/run/tri shops in the area to see if they have
one
you can rent. If you live in a warm climate there may be no reason to buy
one,
however if you live where the waters are often in the 70s or colder you
should
probably consider one

If you are going to buy a wetsuit, make sure you get one that is made for
triathlon. A dive/jet ski/etc wetsuit will not give you the freedom of
movement
you need to swim effectively. Triathlon wetsuits generally range from about
$100
to over $400 depending on the type and quality. There are several types of
triathlon wetsuits on the market: TypeDescriptionAdvantagesDisadvantages

Shorty: No sleeves with short legs
  Cheap, easiest to remove in transition
  Least exposure protection and speed improvement
Farmer John: No sleeves with long legs
  Improved warmth over Shorty without sacrificing range of motion
  Less speed improvement than a full suit, slower transitions than Shorty
Full Suit: Full sleeves with long legs
  Fastest suit with best exposure protection
  Arm movement somewhat restrained, slowest transition, most expensive