Justin Bieber: I'm the Captain of My L.A. Lake House

Justin Bieber turned into a skipper overnight when he signed a lease for an L.A. pad that sits on a lake ... with its own boat and dock.

TMZ has learned the Biebs has been dropping $80k a month on a 10 bedroom home in the tiny Toluca Lake neighborhood. One of its best features -- 16 bathrooms! No more peeing in buckets for JB.

It's also got a gym, game room, screening room and a pool straight out of a 5-star luxury hotel -- and of course ... the whole boat thing. Despite the huge rent check, Justin's barely been home since he's been on tour.

Now that's he's on a break ... it looks like Justin and his pals are making up for lost time. Don't get it twisted though ... we're told he's been a great neighbor. Oh, how times have changed.