Sorry, I have no clue I've literally only spent hours in Tahoe in the summertime.
Are you staying in South Lake? Right at the CA/NV border? That is where most stuff is happening, I think there is a beach right there behind the casinos and I'd assume its pretty good but I have no clue. Usually the partying happens right there on the Nevada side where the casinos are. Thats where they have the concerts, check out who is playing while you are there they get awesome acts and the venue is GA so you get really good seats.
I do know there is a marina and stuff around in SLT more into California so there might be a good beach there.
Don't know anything really about North Lake I go to South so find a Squaw cat to give you tips for up there but I think the north side is supposed to be nice in the summer and theres stuff to do and beaches.