Check it against the boat's bounding box
when you touch the boat, you can get boat.bboxleft (as your starting X point) and bboxright or width for the right edge, same goes with top to bottom.
ex, for a 256 width boat with 4 equal 64px parts
just do on touch/click
ceil( (mouse/touch.X - boat.bboxleft ) / 64 )
to get a 1 to 4 result