"Roll the Old Chariot" is always a good one for improvising, and it's usually a confidence builder for whoever leads it. People always love to sing along with it.

You might look over some of the sea poems by Cecily Fox Smith that have been recently arranged for singing that are posted here on a C. Fox Smith thread. She certainly got the sailor language down and probably had a great time singing some of the traditional sea shanties.

Good luck!
Charley Noble