9 April 2017 Last updated at 13:09 BST
The 80-tonne machine is designed to dig into sand to help lay cables for an offshore wind farm.
It fell into the trench that was being dug and became stranded by the tide.
The people that were operating the digger are safely back onshore.
Now nobody is able to move it for two weeks, until the water level is low enough.
In the meantime, engineers are trying to pump out all the fuel that was in the digger to stop it polluting the water.
A digger has been stranded out at Sea off Worthing beach in Sussex.