The Fourth Element Thermoclines should be fine for the water temps there. Basically a 2.5 mm suit right ? Depending on how cold you get(or not) you might be too warm. I dive with a 3mm full Bare Velocity when I am tropical. I am comfortable with that and I also like the skin protection of a full suit. I dove in October with a couple of guys who wore nothing but shorts and a t-shirt. Bring the Fourth Element and give it a try.
When I dove in Cozumel last, the going rate for tipping the Dive master/boat crew was anywhere from $ 10-20 for a two tank dive trip. I think I split the difference and gave $15 per day. I dove 5 days and paid them at the end. The dive master then splits up the pooled tips with the crew.