Backers of the Disney-style theme park on the Kent estuary site have floated plans to ask builders to live on ships on the other side of the Thames.
London Resort told the BBC the ships idea was under “serious consideration” because of a lack of temporary accommodation in the area where construction is due to stat next year.
After completing their shifts on the Swanscombe Marshes site workers would be transported across the Thames to the ships at Tilbury.
Unite national officer for construction Jerry Swain said: “On the face of it this looks like an attractive option for workers.
“The Hinkley Point project has demonstrated how good quality accommodation for major construction projects attracts highly skilled workers.
“However, it is essential that the cruise ships don’t in reality become an expensive prison.
“It is essential that those concerned in the project enter into negotiations with Unite, to provide guarantees on not just the price of accommodation but the cost and quality of the food provided, as workers will have no access to their own cooking facilities.
“Workers are human beings and in order to protect their mental wellbeing it is essential that they are able to freely leave the ship and visit the local community.
“There also needs to be guarantees about the quality of the accommodation throughout the life of the development.
“At the moment due to Covid-19 there is a huge surplus of cruise ships but when that market recovers, it would be immoral to force workers into clapped out hulks.
“If these concerns are not addressed then the most skilled workers will vote with their feet, they will either look for work elsewhere, or become demotivated which will greatly damage productivity, creating delays.”