A lot of "It's actually very interesting if you know..." here, but I'm pretty sure it's supposed to be a lame reveal. Most plot points and characters somehow call back to previous events, and this is no different. Dude gave his life, killed a ton of people, and is ready to kill again for a stupid boat, or for a man who's already on his death bed. It's meaningless either way. This reflects a lot of characters who attempt to bury the past, but it also reflects Kiryu in a way. He fights to defend the Tojo, but the Tojo's been dead in the water since 3. He's giving his life for something that's ultimately meaningless. While the previous final Kiryu bosses were typically about the Tojo clan, we don't fight Iwami for any Yakuza reasons. It's just for Haruka and Haruto.

TLDR; The boat is stupid as a way of saying Kiryu is stupid. Or maybe I'm defending a dumb plot point, I don't know.