We are avid divers and bring our own gear. We wear our full 3mm wetsuits for two reasons.
#1 when you dive 2 x a day for 5-6 days in a row your body temperature drops so this keeps us warm
#2 for protection from scrapes and any stingy things
I don't think #1 will be an issue for you doing just the resort course unless you fall in love with diving and switch it over to the full PADI certification course.
#2 can happen any time...even to swimmers off the beach. A rash guard or t-shirt should work fine if you don't want to rent a wetsuit.
Cindy: You're gonna love it!
Couples Negril - September 2011, 2012, 2013 and have just booked 2014
Scuba Club Cozumel - Jan, Apr & Nov 2012 and Jan & Nov 2013
Cobalt Coast, Grand Cayman - May 2013
Retiring & moving to Cozumel, Mexico in early 2014