Behind a house is generally the cheapest because it is no frills, but homeowners usually are more concerned with a fixed dollar amount for their property than they are the size of your boat. They usually quote you a price and tell you the max size boat that fits there. If you have something smaller, you still pay the asking price. Usually it includes use of the hose for a washdown when you come in, and basic electric if you need it.
I have a friend that rents 50' of bulkhead behind a house in Massapequa. He pays $2000 for the year. His son was renting a spot a few blocks up the canal for his 19' boat. The owner of that house rented his bulkhead out in two sections to my friends son, and another guy. They each paid $1000. Some people like to do it that way so that they don't get big boats.
The guy that rents the slip in front of me pays $3500 year round. It's 50' of private property, but not behind a house. He keeps a 34' boat there. It is a nice set up with water and electric, but none of the other amenities you mentioned in your first post.
I keep my boat on private property that I own, but if I weren't there, and were looking for a place with the amenities that you mentioned, I would give the AYC in Lindenhurst a real close look. It has great bay access, well protected, and a very short ride to the Robert Moses. They have a pool, bathrooms, security, fuel, pump out, and you can work on your own boat or bring in an outside contractor as long as he is cleared with the office. Not many other boatyards or marinas will let you do that. There are very few floating docks, but I have noticed that a good number of slips there have finger piers which make getting on and off the boat real easy. Slips are frequently advertised for rent by private owners on Craigslist, or through the office for usually a little more money. I've seen several slips advertised for sale there too, and have spoken with one or two would be sellers in the past that wanted to rent their slips until they sold. The problem with that is that if it gets sold, you may have to leave at the end of the season or something.
If you can forego the amenities and just want to try the area out for a season, then renting behind a house might be the way to go. It will be cheaper, and give you the chance to look around the area.