as a simple example, you and your bf now swap boats

the lw boat your bf is in will be deeper in the water than when you were in it >> more drag

his boat that you're in now sits higher in the water >> less drag

the boats are designed for different weights but thats not really what we're looking at here, we want to know how much water is displaced as that determines the amount of water-boat surface contact surface area and hence the drag.

A scull rated to a heavier weight does this by being wider and/or longer, increasing the water-shell contact area [which is the key thing as by spreading your bfs weight over a larger area the boat 'sinks' less and has the right water clearance] and increasing the drag