Draw a freebody diagram, it will explain it really well
the boat is floating on top of the water, which means that the net acceleration in the y direction must be zero
the boat is not sinking (dominant downwards acceleration/force)
the boat is not flying (dominant upward acceleration/force)
that measn
[tex]F_{net_y} =0[/tex]
now, if you drew the FBD, you only have 2 forces acting on the boat.
the upward bouyancy force on the boat and the downward force due to weight
[tex]F_{net_y}= F_{bouyancy} - F_{weight}[/tex]
since the net force is equal to zero
[tex]F_{bouyancy} - F_{weight} = 0[/tex]
and thus
[tex]F_{bouyancy} = F_{weight}[/tex]
