Very interesting thread and nice tests.
When driving a bike or rowing a boat if you turn the debug mode and bear with the spam of other debug messages, you will fish out three stamina relevant messages: Load: Mod: and Burn:
Load is how much the engine strains to keep current speed (0-1000) In muscle-powered boats YOU are the engine, so it will be significantly harder to keep the speed of larger and heavier boats. Whatever in the code affects load, will affect final consumption of stamina. A bike for example at 32 km/h has high load
Mod is the stamina modification that says how load was converted to stamina burn.
Burn is final stamina consumption including some base burn ratio etc. Normal max stamina is 10000.
You may use it in your tests. Good luck and report what you find.