Okay, I tried it on all six boats you can buy at the pier and the option is still grayed out. I tried when the boat was moving, when it was still in the water, with just the sim that holds the Friend of the Kraken reward, with just other members of the household, with everyone in the same boat, and even on townies that sailed past. Nothing.
Everyone is a Young Adult, the sim with the reward is a witch (the only supernatural in the house), and it's currently a rainy day in Fall, if that makes any difference.