There is no one calmest or roughest. The sea can be anything in any month. All depends on surrounding weather systems, winds, etc. Any day within a month can have rougher or calmer seas. All in all May is a nice month to cruise but no one can say if the days of your cruise will be calm or rough. We have had seas as soomth as glass and seas very rough in all the different months we have cruised. Even during the same week!! Have fun and enjoy. I personally enjoy when the seas are somewhat choppy. I find the rocking of the ship very soothing.
MJ