Tried it once. Fitted a 8x4 (inch) airscrew into a 540 motor and trimmed it down until the current reading was the same as a P35 waterscrew would produce with the same motor/battery combination. I seem to remember that airscrew ended up about half its original diameter.
Lots of "ifs and buts" in such a method. For instance do water and airscrews unload by the same amount when the model is running at full speed? The airflow conditions must be standardised too, changing how the air is drawn into the propeller and then blown out could affect the motor speed.
From experience with RC aircraft, a SPEED 400 motor using a 6x3 airscrew would draw 8-10 Amps with the model held stationary. Using the same motors and battery packs in a boat and I'd be reluctant to load the motor with more than half this current with the model held stationary in the water. I know that in aircraft the motor can benefit from some cooling air flow (but they still get hot!) but I suspect the motors greatly unload (i.e. speed up) when at normal flying speed.
Even if an exact airscrew/waterscrew match cannot be made, a good enough approximation might be possible.
Glynn