East of Estes Park (out on the prairie) there are many resevoirs and farm ponds. I can't think of any that would be worth diving. (Later in the year, Don and Dee might want to catalog high altitude pond scum)
Along the front range lakes the visibility would be about zero in May. The runoff from snow melt would have them very cold and muddy. The runoff doesn't even peak until mid June. Estes Lake, right on the edge of town is mostly just muddy bottom. Decent fishing in the evenings July and August. I believe the deepest part is going to be about 15-18 feet. Not many weeds or plants yet in May.
West of Estes Park is Rocky Mountain National Park. Some of the most beautiful scenery in the country. Ranks right up there with Teton, Wyoming and Driggs, Idaho. There are many high country lakes that will be clear enough to dive later in the year but will probably still be frozen over in May.
Estes Park in May is a beautiful area. You will have a great vacation. Baby deer and elk in the meadows west of town. Mountain Goats and Bighorn Sheep a little farther west. Marmots playing in the rocks on the hillsides. One of the best birdwatching areas around at that time of year. Temeratures 70 daytime 25 at night. Probably will snow once or twice but bluebird skies in between.
If you are a Steven King fan, some of his movies were filmed at the Hotel there.
But it is not a dive site.
Tom