I've taken Amtrak from RI to Manhattan several times, round trip. Some on the same day. It's incredibly easy, albeit, not cheap.
For your mission, I would stop in your local Enterprise rental car place and price a one-way rental. The local shop in NJ will come to the marina to pick up the car, if you coordinate. At worst, you drive to the local Enterprise and they'll drive you over to the marina for free. I bet it's no more expensive than all the bus/trains/ferries and much easier. Door to door. I've done it and was surprised how much less expensive it was than I was expecting. It may depend on where they need the cars and which sizes, so talk to them, don't try to book it online.
Great Idea, I use Enterprise, almost exclusively, even purchased a car through them once. Great company. They have a Location in Jersey City. I just priced a one way trip from Boston (area) to J.C. on the net and it's about $154.00. (economy) Might be less expensive than the Public Trans. and certainly more convenient with Luggage. Good Idea to call ahead though and confirm everything. I do a delivery to Eastern L.I. where renting a car one way is much easier than public trans.