SeppoV12, trying to import your boat as a staircase is a good idea but you'll have to go further to get a not rectangular hole. You'll have to include your boat in a SH3F furniture library and a [font=courier new]staircaseCutOutShape#[/font] property in its properties file to define that shape, or add the right property in the [font=courier new]Home.xml[/font] entry of your SH3D file.
Please, read this thread and this document about SVG paths for more information.