I've been told different

[joedog]

Engine performance is irrelevant if the (fixed pitch) propeller is turning the same RPM. Of course normally aspirated engines suffer an open throttle manifold pressure decrement of about 1" of mercury absolute pressure (about 0.5 psi) per thousand feet rise in elevation. They also suffer decreases in performance due to rises in ambient air temperature, and this effect can be significant. Increases in humidity also adversely affect performance. But fixed pitch propeller RPM is the ultimate test (if the propeller is not changed, and if it is operating normally). The suggestion that lower ambient air pressure can cause an increase in cavitation is interesting, but I think it is ultimately unsupportable. Cavitation occurs when local pressure is lower than the vapor pressure of water. The vapor pressure of water at 20 deg C is 2.3 kilopascals, or .3 psi. A propeller operating at 8 inches depth experiences a static pressure of atmospheric (nominally 15 psi) plus about .3 psi hydrostatic pressure (about 15 psi in 33 feet). At 2000' the atmospheric pressure is about 14 psi and the hydrostatic is the same, given equal water density. To achieve cavitation the pressure must drop from the 14=15 psi range to the .3 psi range. I would think that any minor differences between 14 and 15 psi atmospheric would be obscured by the large (hydrodynamic) drops necessary to achieve .3 psi locally. But I would like to see experimental or clinical data.

Hope this helps.
Just messing around with ya, sorry.
My understanding is,
A general rule-of-thumb regarding horsepower vs elevation is that you lose three percent of your horsepower for every 1000 feet gain in elevation. This applies to normally aspirated (not super-charged or turbo-charged) engines. Decrease in horsepower should equate to loss of speed too.
Now I've never traveled great elevation changes with boat myself so I really DON'T know.

Almost forgot, welcome to the site Dadilator!