A digger used for work on the Rampion Offshore Wind Farm is underwater off Worthing beach.
The digger is specially designed to drive in the sea, while keeping its driver cabin elevated above the waves.
However the machine is currently stationary in the sea, with the cabin partially underwater.
“I should imagine it got stuck,” said onlooker Andrew Sharp, who has been following the construction closely from his nearby home in Western Road.
“I came out today and saw how high the water was. It’s come right over the cabin,” he added.
A spokesperson for the Rampion Offshore Wind Farm said: “We can confirm that the elevator excavator working off Lancing beach is currently immobilised and the operating personnel are safely back onshore.
“For safety reasons a guard vessel is on site and buoys have been placed around the excavator. We are awaiting the next low tide to seek to move the vehicle.”
HM Coastguard attended the scene.
There is no indication as of yet how the construction crews plan to get the machine out of the water.
A spokesperson for Worthing Borough Council said they are aware of the situation.