Warmer water to swim in, water going over the falls, warm weather, and no mosquitoes is an impossible combination. The mosquitoes come out with slower water flow and warmer weather. Lots of water coming over the falls usually means the water will be very cold as it is snow melt and the flows in the streams and rivers are deadly. Warm water, warm weather, and no mosquitoes is usually late August thus no water in the falls.
You only get to pick any two or three but it doesn't matter as any where in the park at any time is beyond compare.
Mosquitoes can be easily dealt with with properly applied repellent. You are on your own in dealing with the cold water. Billi-e-g and I measured water at 33ºF in Falls Creek in the spring of 2009.
Old Dude