It's the coldest of all months for water temp:
Weather climate St Martin island - weather forecast for st martin st maarten - Tourist Office of Saint Martin
Thermocline depth I view as irrelevant because you're only talking of a difference of (usually much less than ±1.5°) which may feel like a wall, but in reality, it isn't much in body impact.
Before anyone throws out this-or-that mil recommendations, which are truly a personalized component of your quite mathematical question....
The missing component to such suggestions, besides personal history, is the much more important factor of repetitive dive schedule. Once you quantify the water temp and what it means to you, understand that thermal protection varies in some proportion to the daily dive schedule and surface interval quality.
To offer specifics as to 1 or 7mm suits,
without knowing the planned repetitive dive schedule, that goes beyond a recommendation based (anecdotal) experience on the individual body type, but into the realm of an incomplete equation.
Any concern about thermal protection
should have a required lead-in statement: "
I will have a beanie/hood, but I was wondering..."
Lot more to it, but your question is (mostly) properly effaced in that link.
Do they still allow divers to use gloves?