A private indoor pool does have real amenity value for all lots as opposed to more slips. A YMCA does not improve property values.

Something seems a bit backward here. The purpose of an amenity/pool is not simply to build a facility that might increase property values a little even if no-one uses it.  It is to have something that the current residents want and will use.   Consideration of "amenity value" is irrelevant.  If something in the community is really worthwhile to the residents, then its value in improving property prices will follow.  If the facility is readily available, it won't matter much if it is YMCA owned or belongs to the OPA.

I wonder if the addition of a few docks might have a negative amenity value.  If a buyer of my house is unlikely to be able to get one of the slips, then the existence of the docks is of no value to him.  To him, payment of a premium to buy my property or an assessment to build or support those docks is just a rip-off.  I.e., The supposed "amenity value" would be negative.  It would have positive value to those who don't want or can't get slips only if the dock charges produced a significant profit and thereby reduced homeowner assessments.