July should probably be great. Typically the water is warmest and clearest during this time. I would highly recommend a few things:
1. Stay on the french side, in Grand Case is probably ideal. It's way nicer than the dutch side: it's prettier and the food is way better.
2. It kinda goes without saying, but eat on the french side, they have terrific food (whether it is ribs, which you can get anywhere, really great french food, or even just the fresh baguettes & cheese).
3. Personally, we love Octopus Diving (
Octopus Diving - Scuba Dive in Saint Martin). They're located in Grand Case. The French require this special French certification to divemaster on that side of the island, so you end up with a lot more options. There are locations on the french side off Grand Case, off Tintamarre which is a small island to the north, and between St. Martin and Anguilla. Octopus also does dives on the dutch side.
4. Snorkeling (and having lunch) on Pinel Island is great. If the water is relatively calm, walk across the island and snorkel on the other side.
We've been to St. Martin about 8 times over the past four or five years. I think it is underrated as a diving area. Feel free to email me if you have any questions. I have descriptions of a bunch of SXM dive sites on my web site (
St. Martin | Les Fruits De Mer) as well as restaurants, attractions and such. There are lots of photos, too. And get lunch at Rene's Boucherie, info is on my site. It is amazing.