I just went to Bermuda on the Horizon on May 3, 2003. It wasn't so bad going back and forth. The weather in May was perfect. But you never know what the weather will be.
Keep in mind hurricane season in August and September which has storms in the Atlantic that could go up towards Bermuda. (I live in Miami, so I keep a close check on hurricanes.) So you might want to consider Bermuda for May or June.
I only felt a little sick at lunch on one day, but I ordered a ginger ale and I was fine. I never felt after that.
Why is the seas always the roughest on the formal nights when you are wearing heels? Just a little tidbit for the women.