How to calculate areas
 

Does anybody know how to calculate the wetted surface of a ship floating in waves at each time-step?

Are the Surface Integral or Averaged Area useful in this sense?

Threshold
 

You might try creating a threshold that measures the "wettedness" you need. Once you do that create a field function and call it "one" then make the definition "1". You can then measure the area of the threshold by taking the surface integral of you field function "one" on the threshold.

See how you go with that.