Actually the Bike and Boat Plan is available at multiple marinas.The plan costs $199 for a "family" of up to 5. At least one of the 5 must be an adult. Various age limits apply for teenagers using the water sprites on their own. It is a length of stay plan, not just one week. It is available for sure at WL, Poly, and YC. (BW does not have a marina, but it is a short walk around the lake to the YC). You must be a WDW resort guest, but do NOT have to be staying at the resort where you join the plan. So BW folks can use it at YC. The plan is NOT interchangable among marinas. So if you get it at WL, you can not use it at Poly or YC. Other marinas may also have the plan. I have heard that CB has had it in the past as well.
All 5 members of the family must be listed when you sign up, but they do NOT have to be present at that time. So if you have family arriving later, you can start using the plan before the others arrive. However, you can not drop and add people to the plan as guests in your party arrive and leave. Once you list the 5 people at the beginning, the names can not change.
The plan includes unlimited use of watermice, pontoon boats, and bikes. Surrey bikes are not included. Young children, not on the plan, may ride along in watermice and pontoon boats, but they can not have their own watermouse or their own bike.
If you are staying at the BW and use the YC marina, you will be limited to boating on the lake in front of the YC/BC and BW (Crescent Lake?), the canal over to the Swan and Dolphin lake, and the canal that goes to MGM. Because this area is smaller than Bay Lake and the Seven Seas Lagoon (which are connected by a waterway) over by the Poly or WL, the boats are "governed" to hold down their speed. The result is a slower water sprite experience than is available on the larger lakes. You might want to consider the convenience of using the YC versus the faster ride if you travel over to one of the Bay Lake or Seven Seas Lagoon marinas.
Ralph