Well in this not a national park, that bridge isn't really sinking, over the 6 or 700 years since it's been built, the land of the broads have dropped by about, 6-10 inches ( glacial isostatic adjustment), sea levels have risen by about 8 inches since 1900 after the roughly stable period 1400-1900. So the bridge height is reduced by 14 to 18 inches since it's been built.
The biggest problem with boats going under the bridge is the ever increasing size of boats..not just height, but width, you look at the old woodies of martham boats, they are considerably narrower than modern boats and it's those top corners that stops boats going through.