Kath,
Not been in May, but as Cynical says this is the start of the "Official" rainy season. However, we were there a couple of years ago for the last two weeks of June and we only had full rain on two days (where you couldn't lay on the beach as it rained all day), and a couple of days where we had the 20 minute torrential shower, and then dry in about 20 minutes once the sun was out.
Speaking to the local Maldivians a lot of them have said that the climate has changed considerably over the past 5 years, and they're never too certain of what it's goint to be like. We lost two days to storms in April this year !!
Agree that it is very humid, and there are more mossies about due to this, but most of the islands "Fog" to try and keep them at bay.
Could I suggest that you find an island and then mail them as they should be able to give you a better idea. We have been there and been told that they've had massive storms on the North Atolls, yet in the South we weren't even aware of the bad weather.