Ainsworth is about 260 miles and 5 hours from Pactola Lake in the Black Hills of SD which offers some very good fresh water diving with viz consistently in the 20-30 ft range. It offers 150 ft of max water depth and the main body of the lake is 110-130 ft deep, conseuqently it stays pretty cold year round (it was ice free but 34 degrees today). Water levels are currently down 27 ft after 6 years of drought so some of the deeper dive sites are getting relatively shallow.
The Angastora Reservoir in Hot Spings is about the same distance. It offers the opportunity to spear walleye, but the visibility generally sucks. Water levels are also very low.
Lake Oahe near Pierre SD is about 180 miles and 3.5 hours from Ainsworth. Oahe has some great walleye Spearfishing and visibility can be superb (30 ft) or horrible (5 ft) and can go from great to horrible in about 15 minutes when the wind picks up. Water levels are at near record lows however. This makes the visibility worse than usual as the waves are washing much more silt that normal. Once (if) the levels come up again, visibility should be superb since all the silt on the shorelines will have washed away to depths of 40-50 feet deep in many places. You can also drift dive in the tailrace below the Oahe Dam and that is always fun.
Ainsworth is about 120 miles and 2.0 hours from Pickstown which is along the Missouri and also offers reasonably good diving. You could go a couple hours farther east and dive Lake Okaboji in Iowa as well.
Lake McConaughy in Nebraska is a couple hours south West of Ainsworth and it also offers Walleye spearfishing opportunities.
So there is not much in Ainsworth, but a few hours in any direction will get you somewhere with diveable water.