Welcome aboard, Richard! This time next week, it sounds like you'll be enjoying a magical cruise to start the New Year!
Be sure to check the official Cozumel weather forecast a few days before you set sail, but from what I've experienced in Cozumel, even in January, it was very warm and the water was comfortable, so I can only imagine it will be nice for you as well. Typically the highs are in the low 80's this time of year. Since I haven't experienced that particular dolphin swim with Disney myself, I did a little research with Chankanaab National Park and their
Dolphin Discovery, and found that wetsuits are available there, but for an additional charge. If you're worried about the temperature, you might bring rash guard type swim tops that have long sleeves, as that would help a bit, but I bet the water will be nice as it was for us this past January in Cozumel.
One of the great things about this particular dolphin swim experience is that once it is over, you'll still be able to spend time enjoying everything else that Chankanaab National Park has to offer! There are beautiful gardens there, a beach area, and even a sea lion show! You're in for quite a bit of fun that day, Richard!
Enjoy planning these last few details before you set sail next week and have a wonderful cruise!