Some guys wear their 3mm, I wear a rash guard but I have no problem staying warm so you would be fine in either. I got my Koah from divers direct in Key Largo, I am not sure about Marco Island but in Naples, about 30 minutes from Marco, Scuba Adventures has a lot of spearfishing stuff. You have a while so personally I would try to get one now before you go if you for sure wanna give it a try, you will want to be real familiar with it before you take it in the water. Its not tough to load but if you don't do it just right you are gonna waste a lot of air trying to re-do it.