Four short lines hold two scenes side by side. In the first, a group of people ("we") sit by the sea, looking for "still ground," a place to rest and feel steady. That's what a holiday is meant to offer: a break from pressure, a patch of calm. The sea is an odd choice for stillness, since water is always moving, and the poem seems aware of that. The calm it finds is a kind of hope more than a fact.
Then the poem turns to the prison, where men are singing until ten at night. It doesn't explain or comment. The singing might be joyful, defiant, or just a way to pass the hours, and the poem leaves that open. What it does is quietly pull the holidaymakers' ease up against the lives of people who can't leave. The mood is calm but uneasy, and the poem's last image is the one that lingers.