Downtown Disney is broken up into three sections. Market Place is open until 11:00, West Side is open until 11:00 p.m. - 12:00 and Pleasure Island is open until 1:00 a.m. - 2:00 a.m.
We leave on Thursday and will also be at the POR. The boats to and from DTD run throughout the day, every 15 - 30 minutes. It is about a 20 minute ride. During this time of the year (this will be our 5th time visiting at this time) the boat will only take you to Market Place. You will then need to walk to the other areas if you wish to see them. If you want to see the other areas as well, take the bus, it will drop you off at either Market Place or in front of Pleasure Island.
My advice, take the boat to Market Place. It's a wonderful ride and you probably won't want to do more than Market Place that first night, you won't have time.
If you are arriving at the airport at 8:00 you won't get to resort until 9:00, then checking in time.....you might not have any time to do any downtown disney that night.