Seagulls do not sleep on the beach or in the sea. where do they sleep?

In their nests which can be found away from beaches: on islands, spits, rocky jetties, sea "junk" along a quiet shore, or even moored boats. To keep safe from predators, they don't leave the nest until they are already full size; they remain safe near the nesting site until their primary feathers are grown.

This is only partly true. They seem to 'nest' at night only when they are actually nesting to raise young. The rest of the time (the greater part of the year) they sleep out in the open where they feel safest - inland this could be parkland, even car parks, apparently, and of course out at sea. Here in South Devon, at sunset we observe flocks of gulls coming from all around, gathering above this one particular valley and then, altogether heading off out to sea, presumably to sleep on the water.