The short tour, which we did, leaves from the Westminster Pier (just a little downriver of Westminster Bridge, on the northern bank). There are also longer tours, which go as far as Greenwich - the Royal Observatory (I think that's its title) at Greenwich is well worth a visit, but there's quite a bit of walking involved. Anyway, those tours leave from the pier right next to the London Eye.

I hope the weather stays nice for you - Josh has been lucky.