Orca
Head below deck. Take the exit behind the stairs to the west. Talk to the man behind the counter to open the shop menu.
If you saved 1000s earlier, I recommend buying at least 1 of the last item on the list, Soap (せっけん). Each bar of soap activates a secret cutscene later in the game, for a total of 3 scenes. You'll obtain a second bar of soap automatically, and can find a third in a chest, so you only need to buy one for now. If you forgot to save up the silver before boarding the ship, don't panic: there are enemies who drop soap later on, so technically it's not a missable item.