There are a few possible locations besides Prineville but pretty much all of them require early reservations. I've stayed at both the Prineville Res Resort (with the marina) and now the State Park. Both have their pros and cons but in the end I think the State Park wins out. Better and more spots, nicer bathroom/shower facilities, docks are closer, WAY less dusty, a little protected from the wind. Both locations are pretty serious about their quiet hours.

July would be cool. My family would appreciate avoiding the last minute chaos leading right up to school too.

Memorial weekend down there is highs in 70s, lows in 30-40s and not so warm water.