Yellowstone Lake is a great place to dive; I used to dive it each summer, but now I dive Glacier. In Yellowstone, I've been prevented from getting out of the water because the bison decided to show up. In Glacier and Waterton, I've had to wait to get in the water because a bear arrived.
The Spire Field, on the north end of the lake, can be reached from shore, although it is a 350 yard swim. I prefer to use doubles to reach the field or at a minimum a ST120. Using a boat is nicer, but there is no anchoring. I've found thermal vents in Sedge Bay. For depth you can dive from the old lake yellowstone hotel. You can boat dive West Thumb and look for vents there as well. Plan to dive the east side of the lake in the morning; in the afternoon the wind picks up, blowing from the west, which can make for rough waters.
I've seen many regulators let go in this lake. Be sure to use one that is adequate for cold water. I've experienced upper 30s during high summer below the thermocline; high 40s above it. Drysuits are not a must, but you'll be much better off with one. Don't under estimate the cold water! My friends and I dive with dry-gloves during the summer. August and September are the better times to dive for visibility. July isn't too bad; it depends on how long the runoff lasts. The bottom is primarily silt, so stay well off the bottom to prevent stirring it up. The lake is at 7700 feet in elevation, so be sure to make your ascent rates very slow and your safety stop should be around 11-10 feet.
Fire Hole rivers offers warm diving (77°) as well as swift water. In August and September the water isn't overly swift. In late May or early June it can be rather swift making it rather fun if you like that kind of stuff. There are two places to get in: one is up river from the chute, and the other is enter the shallows then swim towards the chute. The latter option is the easiest, and the former can be challenging if you are not used to swift water. Max depth in mid-summer when the water levels drop is 35 feet.
I've been up there with people who want to make three dives in a day. While it is possible, the vast majority are unable to because they get tired, or it takes forever to drive from one spot to the other. Traffic in the park is horrible (as bad as rush hour on I-635 in Dallas, where I used to live), which is one of the reasons I stopped going Yellowstone NP. The camp grounds are usually kept clean and the shower facilities are rather nice, but it is NOT quiet and many people make the spots a horrible place to get some rest. If you plan to stay in a hotel, Cody, WY is about two hours from the lake, and West Yellowstone, MT is about an hour to hour-and-a-half from the lake, and 45 minutes from Fire Hole River. Wake up and leave very early to avoid the worst traffic.
Plan on some excursions besides diving. There are plenty of waterfalls, geysers, wildlife, trails to walk along, and some good back country hikes to pursue.
