Here is a plan for a miniature shanty boat small enough to be propelled by oar with an optional downwind rig.  It was originally conceived for a minimalist wilderness escape but will work just as well as a floating playhouse or swim platform for the kids.  Construction is the very simplest plywood sheathed with glass and epoxy.

Construction: Plywood over temporary building molds.

Length on deck: 16ft 0ins
Beam: 5ft 6ins
Draft: 7ins

Building time: 500 hours
Skill level: Basic