You have to use that expression to tie the boat center to the null position. Parenting will not work. After you have applied your tracking data to a null and the null is accurately following your eye you are ready to build your expression.
First reveal the position property of the null. Now hold down the Alt/Option Key and click on the bulge center property in the effects control panel. This will start an expression and then simply drag the picwhip from the bulge center property in the timeline to the position property of the null. The expression is complete and the Bulge center well now follow the null's position.
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I tracked the motion and applied it to a null. What I was going to do was add a bulge effect to my eyes, and parent the bulge center of that effect to the null to make it move with the null, thus following the movement of me and the movement of the camera to make it look realistic. However, it seems you can't parent certain effect parameters (like bulge center) to a null object.
parenting only parent's transformation properties of position,rotation and scale. it does not parent opacity or any other parameter. Rick suggestion is the way to go if you want to "parent" anything else.
more about parenting here:
more about expressions (to link certain properties together) here:
Did you ever solve this issue?