Update About Boats:
I double-checked the DMG (pp.53ff) after last session, and it looks like the capacity/size numbers there are much more generous than in the WSG — and more in line with my gut feeling. Here is a summary of the facts for rowboats and large barges (I will add these to the vehicle section of the House Rules document, too):
Rowboat, Large
Capacity: Up to 10 Human-sized
Hull Value: d4
Length: 8′-20′ (The one you used was 20′)
Width: 2′-4′ (The one you used was 4′)
Average Speed: 2mph (3520 yards; 58 yards/round)
Barge, Large
Capacity: Between 20 and 100 Human-sized
Hull Value: 2d4
Length: 25′-45′ (The ones the Bargemen have are 35′)
Width: 12′-20′ (The ones the Bargemen have are 15′)
Average Speed: 1mph (1760 yards; 29 yards/round)
While editing the House Rules document, I also adjusted the carrying capacity of the carts/wagons. My new thinking is that the average character should be assumed to ‘weigh’ 15 slots (5 quick item + 10 backpack) and the new vehicle capacity numbers reflect that assumption.
Re: Carts. Definitely seems much more fair that way. Since it has 48 slots, with out reasoning, it could carry 8 people. Even with me arbirtarily limiting it to six, 3 seems much more realistic.
The slot allotment for the carts has changed slightly, though, in an upwards direction. It is now 90 for a medium cart (after some back-and-forth on my part), which equals exactly six people under the new rules (or less + other cargo).