Conditions:
Mosquitoes: They're out in full force.
Swimming: I have reasonable cold water tolerance...by that standard, the water was swimmable. This is probably the earliest I've been swimming in a Sierra lake (previous 'record' was third weekend of June in Lake Tahoe). The thermocline was probably about 4 feet down, to be really good for swimming it needs to drop a couple feet. Treading water - my feet and calves froze. Swimming, floating on my back...I had fun.
A morning view of Ostrander Lake:
Two views of the Clark Range:
A meadow near the trailhead: