I probably wouldn't leave them at the pool, but I would have no problems renting them a movie in-room, getting them a pizza, and heading out. I would assume that at least one of them has a cell phone and a level head on his/her shoulders. They can call if there are issues. My pool issues revolve around water safety and not having someone would be watching out for them. Yeah, I send them to summer camp alone, but I know that they have lifeguards specifically watching all the kids in the water. You can't assume that at a hotel pool. And 12/14 is still in the "watch me! I'm gonna do something insanely stupid!" phase. Just my luck, one of them would cannonball onto a 3yo kid or run and slip and split his head open, yanno?
As long as you are ok leaving them while you are at home (that's about the age our boys were when we started leaving them and driving 20 miles to the nearest "big" city for dinner alone), I would think they would be fine. Our guys are 16 and 14, and we are leaving them at WBC for at least one evening while we go to CityWalk for an adult-only dinner at Emeril's or maybe Tchoup Chop at RPR. They know the rules, they have phones, they will be fine.
Yes, this is a family vacation, but Mom and Dad deserve a little pampering, too.