I have cruised to Cozumel in May. Expect some showers but they usually pass soon. Hot, hot, hot weather. Bring plenty of sun screen, lip balm and your sunglasses. Wear cool comfortable clothing. Bring at least two comfortable pairs of shoes. I say this because I learned the hard way; I got caught in a shower in the Bahamas and had to wear high heels all weekend. Bring a light jacket or sweater because the air conditioning will feel cold after being out in the heat.
Enjoy your cruise!