Spagnum is probably not the best stuff to plant your plants, unless you are using meat eating plants for two reasons: 1.your plants will sink into the moss and might kill your hermit crabs. 2.it is hard to landscape because of the last reason. Also, your hermit crabs, because they are herbavoris, will eat your plants.
The average daily low temperature over the last few years has been 77 F. The mean daily temperature in Bermuda in March has been 80 F over the last few years. The average sea surface temperature for the month is 83 F, almost the warmest it will get all year. Expect around five inches of rain.
There is no monsoon or rainy season because Bermuda is not in the Caribbean, but some months tend to be wetter than others. But others can be devoid of much rain. Also, several - like late June, July and August in particular and early September - can be extremely (and unpleasantly) humid. If this affects you badly, and you don't mind cooler water for swimming, then try May or June or late September to October. For weather forecasts, check the Bermuda Weather Service page.