It will be dang hot!
Being from Texas, I love it like that! All the clothes you need is flip flops, shorts.. and a light colored t-shirt. I would also recommend a hat.
I stay pretty tan in the summer due to outdoor activity here in Texas. It helps keep me from getting sunburned too bad if I go down for some summer diving. I always try to use a t-shirt or a shady part of the boat for my sun protection instead of sunscreen lotion. Sunscreen can come off in the water and it's not very good for the reef.
As far as underwater exposure protection in the summer, it's more for protection against rash or stings than for warmth (although I don't touch the reef, better safe than sorry). Any heat you lose diving will be gained back during your surface interval, which won't quite be the case if you go in January. I also take a wide brim, lightweight boonie-type hat to keep sun off my face and neck (again, it cuts out the need for sunscreen). The water temps down there stay pretty constant. 78 - 81 degrees, give or take.
Being from Texas, and especially after living in North Dakota for a couple of years when I was younger, it can't get too hot for me, but if your from Canada, it might take you a little while to get used to the heat so be mindful of the heat and humidity and dress accordingly, don't over exert yourself outdoors, stay hydrated,.. and I always jump in "big blue" whenever I get a chance whether it's diving or just hanging out at the beach... although a pool will suffice.
The nights will most likely be warm (as in like 80 to 90 degrees or so), but with a breeze and no direct sunlight, it will be quite pleasant....it's the direct sunlight that'll get ya, so try to avoid being out in it for long periods of time without a place to cool off.
There's nothing I love more than roaming around San Miguel dressed only in shorts, t-shirt, sandals and my floppy hat and sunglasses...true beach bum attire!
Dang! Every time I answer one of these questions on the board I start jonesin' for a trip down! Man! I love that place!......well, I'd love to stay and chit chat, but I'm off to the Funjet website to peruse the deals!