Normal has changed between 1970 and today. Males averaged 166 lbs now average 194. Females once averaged 140, now 164.
So at minimum it's an additional 96 pounds per row of additional weight displacement above the 1970 designed specifications that is in play.
Simple to calculate, you pick a maximum weight you want support and it's exactly the same amount of water you will displace.
Draft height in feet = gross weight/ (length x width x density of liquid it's suppose to float in.. for fresh water it's 62.4 lbs per cubic foot.
The physical channel hasn't changed in width since it was built, so the only variables are the guests weight, the number of boats in the water at once, and the starting depth before guests are added.