She's afraid he'll be wanting to go to sea later on and she doesn't want the notion encouraged in him.

But what could I do, Mistress Blythe?
I'd PROMISED him, and I think it's sorter real dastardly to break a promise you make to a child.

Come, sit down.

It won't take long to stay an hour." The wind was off shore, and only broke the sea's surface into long, silvery ripples, and sent sheeny shadows flying out across it, from every point and headland, like transparent wings.

The dusk was hanging a curtain of violet gloom over the sand dunes and the headlands where gulls were huddling.