[killarney_sailor]

Since you are on a river you can have a pursuit race with a difference. You need three marks, with the start in the middle (perhaps two actual marks forming a gate) and an upstream mark and a downstream mark. You start at your time, but you can go choose to go upstream or downstream first. You need to round both the upstream and downstream mark and then finish. Boats are finishing from both directions, in theory at the same time. You choose your route based on traffic, potential changes in the winds.

 

[killarney_sailor]

Maybe a real navy cockswain can help out here. I once observed a navy competition of precision boat driving. It was much like a poker run, but there was no chance element and speed was the ability to maintain precise speed of your craft, differing for each leg.
You could turn this concept into a boating rodeo where several seamanship skills are involved going to and arriving at precise locations at precise times, tacking drills through a chicane, etc. disallow GPS and plotters - compass and chart only. You could have many stations and send boats out in different patterns, maybe pairing like boats. By putting precise speed vs max speed into the equation you could have all the boats returning as a fleet, raft up and party while the judges tally the results.
A varient would be instrument sailing, where the helmsman follows course and speed instruction from a navigator who is blinded to the outside world. That might be too difficult for the size boats you're talking about.
Just thinking...
John

I think this is called a predicted log. I am sure there would stuff on the internet about it.