Buses start running to all parks, from all resorts, by 6:30 each day. You may find the MK buses starting earlier than that...sometimes by 6:15ish.
SO...if you have an 8am ADR at the Polynesian, you should be out at the MK bus stop at your resort by 6:30. As already noted, these buses don't come quite as frequently as the regular buses. So if one is just leaving as you get to the bus stop you may have a longish wait, up to 30 mins. Most likely the wait will be less than that, but better to be safe than worry. When you get to the MK you can hop on the monorail..it starts running around 7am. You should plan on it taking about 15 mins to get to the Polynesian from the MK stop.
The other thing you can do is to ask the bus driver, when you board, which is faster to the Polynesian....boat or monorail. He/she may offer to drop you at the Polynesian on his way out of MK. He may not but it's worth a shot.