Guernsey's about the worst lake to sail: shallow, hot, buggy, and (as you note) essentially drained dry every summer. The draw-down usually begins around July 4th, but in these drought years it was never full to start. A bigger, deeper, and better reservoir is Glendo, just north of Guernsey. Went there this spring, when it was very full. Had a good time, though five busted ribs cast a pall over the weekend. Lots of powerboats.
Central WY: Seminoe, Pathfinder, and Alcova. Seminoe is classic desert canyon reservoir, snaky with hard bottoms and lots of inlets. Pathfinder is a wider, rounder lake; it only has one real ramp, but it works even in very low water. Minimal boat traffic, nice beaches, clean western shore so the winds just HOWL across that lake. Beam reaching heaven, 8 miles long in good times. Alcova is the smallest of the bunch, but it is the most developed and has a very active sailing scene. Biggest fleets are J22/U22 and J24, most sailing out of the Casper Boat Club
Alcova is only about 1x3 miles, with an island in the middle, and it's snuggled in a little basin -- so the wind swirls like hell. And the number of soused Casper speedboaters .... But it is always full of water, so that's worth something.
Fremont is our goal this summer: narrow, ten miles long, very cold and deep, and nestled below my favorite place on this planet -- the Wind River Mountains.
Wind is less reliable than the Plains lakes, tho. Nearly all Forest Service land, so you can camp & walk as you please. It's Pinedale's water supply, so observe the sanitation guidelines.
Yellowstone is the biggest, and a very serious lake in some ways. But you gotta wade thru Park Service red tape to sail on it, and camping, anchoring, and so on are tightly restricted. Also, the best launch is on the wrong side of the fishing bridge. Jackson Lake is also Park Service, more or less, but the Teton rangers are a MUCH cooler bunch to deal with. Watch for aggressive grizz on the western shore.
Other lakes: Boysen, Half Moon, Fontenelle, and Flaming Gorge.