Well, as an estimate, note that a keg of stout is 4-weight.
A keg (in real life) weighs about 160lbs.
I would also assume an adventurer weighs 140-200lbs (not counting gear)... this is for a human male, of course.
Also, keep in mind load is not just weight, but how awkward/difficult something is to carry. But a boat or whatever is specifically designed to carry people.
So I'd say a "normal" adventurer is about 3-weight (not counting gear). If they're particularly big or heavy, 4-weight. If they're really light, maybe 2-weight.
So a barge can carry about 5 average guys without gear, or maybe 3 with. That sounds roughly - but maybe not perfectly - accurate.
On the other hand, maybe passenger weight isn't meant to come into it, and "load" is just literally how much stuff it can carry (plus however many passengers make sense).
So a donkey + cart would carry 20 load of stuff plus no people, a horse or warhorse would carry 10/12 load of stuff plus one person, and so on.
In this case, you'd need to just eyeball how many actual people can enter a boat.
Anyway, none of this is official, but maybe it helps some...
- Alex