Is it an operational reservoir? Please be careful, if they're abstracting water the intakes may only be a few feet below the surface and not necessarily visible. The intake pipes could come out the dam at varying depths to allow for the drop in water level or temperature or algae blooms at different depths.
You don't want to get stuck in an intake.
If there is a tower in the middle that probably Has the intakes on it so don't swim around it.