Congratulations with your wedding... you will love it, the water in Cancun is so blue! Makes for the perfect back drop.
I would highly recommend having your wedding later when the sun is setting than earlier. You have to remember that by 11am... the sun is already HOTTTT! At least in the evening the heat will be coming down and usually getting an afternoon breeze. Don't worry about your day going by too quickly! Once your day comes... sit back and appreciate every moment of it. Let go of all the details and let those around you take care of you!
Part of the reason for having a destination wedding is that you can relax here. If you want to micro-manage everything... do that... but the day of your wedding let it all go! Have fun and try and take it all in the way it comes at you!
Best of luck!