Don't forget your hat and your suntan lotion
One year I was snorkeling in Cozumel in December. I went out a couple of times for one half hour each time. Oh boy - did my head get sunburned! I had put on suntan lotion everywhere else- but I forgot my head!
Even though it is winter, and even though you may be in the sun for a short time - you can get quite sunburned the further south you go.