The Peel inner harbour is being shut while maintenance work is carried out.
The area will be closed to vessel traffic during the high water period on Wednesday night into Thursday morning.
The Department of Transport is to replace the bottom seal on the Peel Flapgate. "Stop logs" will be used to retain the inner harbour's water level.
They will have to stay in place overnight, which is forcing the closure, the government said.
The pedestrian access will also be temporarily closed to allow work to be carried out on the bridge lighting.
It will be closed from 1300 BST on Wednesday to 1800 BST on Thursday.