I think the definition is somewhat flexible.

What would you call a surf or kiteboard instead?

Disclaimer: being subjective here and not too attached to the opinions, I waiver between words having independent meanings or defined by uses (a more Wittgenstein-esque approach, at least at my limited level of understanding)

I think that those could be grouped together but would require a new category that may or may not include water skis or wakeboards. Another condition i was thinking about placing on boat was a steering mechanism akin to a rudder but then realized rowing shells often don’t have those although I’d consider them boats.