I am definitely not the frat boy type (and never have been) but I've always enjoyed staying in South Lake Tahoe CA / Stateline NV versus elsewhere around the lake. The south shore offers far more restaurants, more shopping, and a busier night life. You'll see people of all ages there.
As for water activities, check out any of the marinas or outfitters in the area. There is *plenty* to do on the water in the summer. And don't overlook the ski lift. The Heavenly lift, which starts right in the middle of the action in SLT, runs during the summer to take visitors up to the rim. It'll be dry and clear (mostly) up there 3,000 above the lake but still a gorgeous sight!