Lands End have great long and short sleeve rash guard zip shirts that have UPF 50. We have a few of those for snorkeling, swimming and wearing when cold.
We have been down there 3 times and have never needed or wanted long pants. We are from Texas and are used to the heat.
I wear a light cotton jacket on the plane in case I need one down there. Invest in some good Keens sandals. We wear them on the plane and when we get off the boat. They protect your toes!! I buy them thru Zappos, 2 day free shipping and free returns