When is the 'off' season for snorkelling in the Maldives.?

Depends what you want. If you want super visibility, go in jan-feb-march, when there is less plankton in the water because less rain. But fewer fish, and fewer chances to see mantas or whale sharks while snorkelling. In July august september, there are fewer people, but much more chance of rain. Less visibility but more fish. In between those times it is the two monsoon changes. More wind, storms, rain. Less fun. Which resort you chose doe smatter. Some beautiful resorts have amazing blue lagoons but no reef so no coral or fish, and the snorkelling has to be done on excursions. (for example Soneva Gili, Rihiveli, etc). Some have incredible reefs, like Mirihi, or the Park Hyatt or Kandholhu. So chose well or you could be disappointed in terms of snorkelling. Also depending on which monsoon you are in (there is the southwest one an dthe northwest one , or the changes from one to the other. You will always have more visibility or less visibility depending on which side of the atoll your resort island is on depending if you are on the wind side or not. And different kind of fish life relating to that.

I would vote for Summer as most travelers come to Maldives from Europe during winter.