The one time I was there, the boat was running until 2 AM. Another time, it was only until 11 PM. I guess it depends on the season.
Either way, just as an FYI - the boat from POFQ only goes to the marketplace side, not the West Side where DQ is, so you'd need to walk from the Marketplace to the West Side and back.
Does anyone know if the DD buses are back to making all 3 stops now? Marketplace, Pleasure Island and West Side? I know they weren't stopping at West Side for awhile...the only reason I ask is if the bus doesn't stop at WS anymore anyway, then it really doesn't make a difference...