In reply to JuneBob:
30 ft is tiny for 25 days?
Wow... there's guides on line some of the Airforces produced for air crew at sea. basically all different dynamic exercises. That's about as good as it will get. But 30 ft won't give you much space, nor will they like you jumping about if you are doing sleep shifts..
When I worked at sea for periods I worked on a boat big enough to throw a turbo trainer up, so I'd set it up on the bow and ride for an hour at a time.. you felt like you were powering the boat.
The other thing was jumping off for a run and meeting the boat down the canal? I did that for 40 miles through the caledonian... nice day out..
But I'd really plan food etc so when I hit land I could run for as long as possible in those days.