well for what its worth, boats have got bigger, therefore they displace more water, now archimedes proved when he got in his bath and displaced an equal volume of water to his mass, then leapt out of the bath and ran through the streets yelling eurika, that a body displaces an equal amount of water, thus it is obvious that the blame lies on all of the larger boats - they now displace more water causing the rise in water levels.
At this point I will gracefully add that I have refrained from leaping out of my bath and running through the streets yelling anything (mainly as i am sitting at my desk at work, admittedly).
(now lets see someone disprove that!)
under this theory, if all of the large boats drilled a hole in their bottoms, they would displace less water, and would once again be able to pass under Potter heigham bridge.