As a boat owner, with my boat in a marina not far from the same area. I don't see any one dumping the bodies from a boat for the following reasons.
1) It's not that easy just to bring body parts to your boat to throw over board. First you have to park your car then load up the wagon from the car then wheel the wagon to the boat, etc etc. Blood could leak from the bags or whatever onto the docks, into the wagon.
2) The marinas are busy places where you know everyone, people would offer to help with your bins and heavy black plastic bags, marinas are open 24 hours a day boat owners have keys to come and go to thier boats, there is a lot of activity in the summer months.
3) My marina is also a Yacht Club with parties going on and people hanging out on the small grounds using the BBQ's, playing cards, socializing whatever it may be. Most marinas you have to walk through the main area to get to dock where your boat is.
4) There is always a tender on the grounds that watches the boats and bring people back and forth who chose to have thier boats on a mooring. There could be a security guard as well. Some marinas also have security cameras. The boats are worth a lot of money from 100k up into the millions. These places are watched one way or another.
5) If someone hires a boat to use they still have to go to a marina and quite often they pay for the captain to operate the boat.
6) If there is not a captain the owner of the boat that is renting knows who they are renting to, its usually not a full cash exchange, (maybe partial) there is a background check and payment through checks or electronically. The person also has to purchase insurance to cover the boat they are renting usually through the owner. People don't just rent thier boats to any joe schmoo. Especially ocean boats can be quite expensive even the smaller ones.
7) It's all not so cut and dry.
Someone using thier own boat or hiring one would add more chances and steps in getting caught then just having the bodies in thier trunks or trucks and dumping them. These are not LAKE MARINAS in the middle of no where, it's an entirely different thing down here in this part of NY. Even if there is a dock at the house, then your neighbors will have docks, summer is a very busy time.
Fire Island is on the map below in respect to Gilgo, or Oak. Zoom it to see the details.
Someone could have driven across the Robert Moses causeway .......making a right dumping the bodies and heading back on the causeway, there are no tolls on the causeway. Maybe cameras?
https://www.google.com/maps/@40.6398398,-73.2908053,12z
