The water taxi would seem a natural extension of Rockland's enhancement of the waterfront as both an amenity and a source of commerce for it's merchants.
As such, they really shouldn't expect to get full reimbursement for the incremental cost of the service.
If you had to take spend $20 to take you and your mate to- and from- shore for dinner (PLUS a tip, cheapskates!), then you'll be modifying who is using the waterfront.
Now, if they made it complementary to those taking a mooring, like some other towns (i.e., included with the cost of the mooring) -- and the moorings weren't crazy, maybe that's different.
It's frankly just trying to find
o 'What will the market bear?' or
o 'Here's a vendor contract that we can use as an isolated "income source" -- let the vendor set the price, we don't care...