Remember the human powered flight challenge of the Gossamer Albatross? Create a water crossing challenge with a W/Kg challenge of crossing the English Channel. A 22 mile challenge where you have to keep your Watts above a threshold.
How would you keep it from becoming boring?
One of the challenges of the crossing was the headwinds. The lower the rider got to the water the more headwinds came off the ocean. The incentive to keep your watts/altitude high was a big deal (you could show the drafting bar as an indication of headwind). Keeping the Albatross out of the water would be the big incentive. If the glider touches the water the challenge is failed.
The challenge could take the riders base W/Kg normalized to set the challenge. For example if my W/Kg is 2.45. The challenge would be to maintain that for the 22 mile crossing.
Another way to keep it interesting is to allow the cyclist to use the standard awards (featherweight, Aero) at intervals of the journey. If you want to make it more interesting you could place the bonuses at altitude so the rider would have to exert more energy to attain them.
If the challenge were significantly difficult to complete and it becomes a bragging right for finishing. That may compel people to add it to their ride options.
There was an old game by Namico called PropCycle that had some interesting ways to keep it interesting.
The entire challenge might just be one where you turn down a road with a dock at the end and attempt to make the distance as a part of a regular workout. If you fail you end up back on the dock and continue your ride.
I’ll give it some more thought…