Summer months (July and early August), Spring (after Easter but before Memorial Day) would be the easiest. Holiday weekends are bad, as is 4th of July week. Your odds increase if you book at 11 months and go down once your reach 7 months. Mid-September thru mid-January would be the most difficult to book. A lot will depend on whether there is a Poly owner who will rent their points to you at 11 months or barring that, anything available for your stay when the 7-month booking window opens for other DVC owners.
I don't think it's too hard to get a Poly room on rented points. However I think this is largely because Poly DVC rooms do not offer something particularly over and above a Poly hotel room. Unlike at BLT or others where you can get larger rooms, Poly rooms are about the same as regular rooms but further from the feature pool. If you catch any deals like the "Sun & Fun" that's going on now, a Poly hotel room will cost you only a little more than renting and have none of the headaches plus book any time, pay later, cancel any time, and make changes up to your travel dates. This would give you the ability to book your land around the sea since the sea dates might be less flexible.
