Yes! In fact, swimming in Diablo Lake is one of the best ways to cool off on a hot day. However, there are a few things to keep in mind. First, the water is cold - even in the summer - so be prepared for a shock when you first jump in. Second, the lake is fed by snowmelt from the surrounding mountains, so the water level can fluctuate throughout the year. When swimming, be sure to stay within your depth limit and never swim alone.