1. The African Queen: Obviously the most legitimately nautically-themed Katharine Hepburn movie. Also an excellent simile to employ when there are bugs. "It's like The African Queen in here!" you say. And then people know you're a serious, but witty person.
2. The Philadelphia Story: Did you catch it? The boat (The True Love) is a metaphor. For the perfect love shared by Katharine and Cary Grant.
3. On Golden Pond: Two loons on the lake, you guys. They return every year! HMMM.
4. Woman of the Year: At the end (spoilers!) Katharine chooses Spencer Tracy over a prior committment to launch a ship. WHAT? IT COUNTS.
5. Summertime: It's set in Venice, so there is both a sexy water taxi ride, and Katharine falls into a canal. But with panache, because she was extremely athletic, like a mid-century Jessica Biel who could also act her face off.
6. Suddenly, Last Summer: At the end (spoilers!) Katharine tells Elizabeth Taylor they should just get into the boat with the captain and forget all this unpleasantness. There is a great deal of unpleasantness, but it was so carefully pruned by the studio that you can barely tell (gayness, apparently.)
7. Rooster Cogburn: Look at her. The next time you think about popping your collar, say to yourself: would I withstand gunfire with that kind of ebullience? What about the rapids (boat)? If the answer is yes, pop away.