Stick with the open heel fins and boots. I had a bad experience (in Cozumel coincidentally) when I was wearing full foot fins. The no-slip coating on the ladder on the dive boat had worn away and because I had bare feet climbing the ladder, my one leg slipped through and I landed hard on my leg. An inch further and I would have landed on "the boys." That was the last dive trip I used the full foot fins. Nobody with boots was having any slippage issues.