Water + Buoyancy

[zell2002]

Following this :
https://www.iforce2d.net/b2dtut/buoyancy

But not sure how I go about doing the : Area and centroid of a polygon, using the Intersection of two polygons

ive found :
http://love2d.org/wiki/PointWithinShape

And assume the func I want is : GetIntersect()

Has anyone done something similar, or any pointers for me ?
Basically just want me a body of water to float around in

Cheers guys!