Check out the two marinas seen in the photo on this link. Another place that rents boats is Callville Bay Marina. The two party places on the lake are Little Sandy Beach northwest of Calville Bay and the Wishing Well in the narrows.
Water leves continue to drop so even those that know the lake well need to be careful. You should also check out the back side of Hoover Dam if you make it out on the lake. Let me know if you need anything while out here in Vegas.