I haven't snorkeled in about 40 years - since then, I noticed that there are now split fins versus full-foot fins. What's the difference? I'm going snorkeling in Tahiti. Also, I hear that you need booties in some of the lagoons there because of spiky coral, etc. Do you think 3 mm. would be enough to protect my feet? AND, when your booties just mention one size, does that default to male size? I wear 8-1/2 mens and 9-1/2 to 10 womens.
Many people really swear by the split fins which provide more thrust through the water and result in less leg fatiguing/cramping. Blade fins (the one piece fins) are just fine for snorkeling if you are a bit more budget oriented. 3mm booties are needed to wear the open heeled fins and will be plenty sufficient to protect you from corals/stinging things :) For you, I would recommend the medium/large. Have fun in Tahiti!