The boat to CR starts at about 7AM (from the CR) so it might be tight.
You could also take the bus to MK (if they are keeping the current schedule) or to the TTC ( if they switch back) and take the Resort monorail to the Polynesian.
A little over a year ago we were staying at the WL and had a breakfast PS at 8:00 at Fresh in the Dolphin. I asked the question about how to get to the Dolphin and I was told by Tyler, a WDW bus driver on these boards, to go to the bus stop at the WL and ask the MGM bus driver if he could drop us off at the Dolphin. The bus came to the WL bus stop a little after 7:00, I asked the driver if he could take us to Fresh and he took us to the front door of the Dolphin. Worked out great and the driver couldn't have nicer. The Poly is so close to the WL I would think something like this is possible.