Ryanair offer travel insurance policies as an add on to a flight booking, or they can also be purchased as stand-alone products. Ryanair travel insurance is offered in two levels, so you can choose the option that's right for you and your wallet.
Jamaica's rainy season coincides with ours here in Florida (I am in the Tampa area) for the most part. We have anywhere from 30-50% chance of rain daily until hurricane season is over. I didn't mean you have rain every day, all day from May to December in Jamaica, but I assume it is a lot like here in Florida.
Although December is mostly sunny and warm in Jamaica, the area is prone to a bit of precipitation during this month. It rains 42 percent of the time on Dec. 1 and 37 percent of the time on Dec. 31, and the average precipitation for the rest of the month is between these two numbers.