Depth and water temp should not matter, just make sure you are buying a scuba mask – some places have some cheapo snorkeling masks that are not made to withstand water pressure. Also some free diving masks are smaller, and not quite what you'd want.
Fit is most important, make sure you try on several – even after one that you feel fits just fine - to compare. Ask if you can try the mask at home/in water as often the mask feels/seals fine in shop/air and then leaks in water. Sometimes shops have student/rental masks that are same models they have on their shelves to try. Try to get someone to assist you in buying (either from the shop or diver friend) who actually knows how to check if a mask fits. (About any mask fits when you really suck it on, and there are some sellers who will not assist you any further than that!)
Comfort can be affected by several factors. Check things like the bridge on the nose pocket, does it press you? What about the skirt above your nose, does it come too low? One major thing that varies by masks is the softness of the skirt silicone. Some have way more supple silicone than others and as a result feel more comfortable - and often also seal better. And talking about the skirt, you also want to decide whether you want clear or black skirt (you can find dozens of threads discussing the benefits of each here).
Another choice you need to make is between purge or non-purge mask. There are people who find that masks with purge make equalizing a breeze but there are also comments that it can be harder to clear with the hard piece of plastic on the end of the nose pocket. There is also the opinion that the purge is an extra failure point as it introduces one more leak point to the mask.
Also look at the buckles and straps. Major nuisance if they are crappy or hard to adjust. You want to be able to handle them while in water too, not just on land, so it’s important buckles are robust and easy to operate.
Last but not least, the lenses. There are lots of styles. Some masks have multiple windows, huge lenses down to your cheeks or just small more traditional looking lenses. Do not fall victim for the look over function. See what fits you, and decide based on that. I personally like masks that offer some added downward view (easier to see down towards your belly button) but it’s not worth having additional lens space fogging up if you need to be clearing up all the time because the coolest looking mask did not fit right! The choice between one window or two separate lenses is personal preference.
If you need prescription lenses make sure you inquire which masks might come with drop-in lenses as it might save you good chunk of money.
Hope this helps.
