I am verifying the following but believe that any monetary contribution towards the maintenance of the lake is not mandatory. Here is a helpful link to Groton Lakes Association that may be able to further supply you with the answers you seek.
The property is bank-owned and will most likely require either cash or a re-hab loan. You will not be able to finance conventionally because the septic system has failed and it is a buyer expense to replace/repair.
If you'd like to tour the property, don't hesitate to contact me.
Broker MA & NH
Integrity Residential Brokerage