Public transportation is your problem - it is really limited on LI to RR stations ( or limited bus routes which take forever to get around.) Why not rent a car for a day? Compared to RR tickets it may not be that much more and would open up a lot more options. Also, the walk from the Port Jeff RR station is downhill to the harbor, the return trip is up hill and I doubt most people could do it in 15 minutes - what will you be carrying?