After reading replies from bdfd, hansmex and Puybaret I tried to use staircaseCutOutShape# property and SVG path to create waterline-shaped cutout of boat on ‘water’ surface.Results can be seen in screen captures below.
My SVG cutout is not perfect and bottom can be seen between the boat and water on right side.
and from another angle
SVG became familiar in some extent but my "water" (=floor) is not transparent as I want.
Now Daniel presented elegant solution for transparent water.
I will try that Export to OBJ and import as furniture too.
Thank you Daniel (and all others)!
