Depends on whether your GM wants to trivialize gate breakdowns and allow mass-movements of people and cargo, or not.
If you can do it with a boat, why not with a gigantic wagon?
If you can only do it with a boat, people would create giant pools just for "shipping" as much as they could through gates.
A one-roll-per-figure gate averages 216 figures per breakdown. That's enough for a battalion of men. But if you can put them in giant containers, then you could pass 216 SHIPS per breakdown, or 216 giant wagon-passages full of troops.
So it would mean essentially you want your entire known world to effectively be in one nexus of possible instant movement, which means distance would no longer be an interesting barrier distinguishing places and trade goods and separating rulers' domains.
Great if you want your game's power and trade and situation to be all about the mind-boggling power/culture/trade collision that would occur.
Otherwise, I'd suggest that if something larger than a one-hex figure passes, there need to be a proportional number of breakdown rolls... say one per figure and one per 500 pounds for large things. Figures riding on a large thing I would still count as figures.
A barge the size you describe is
probably in the neighborhood of 100 tons so that's 500 breakdown rolls plus one per figure or thing aboard.