O, well for the fisherman's boy, That he shouts with his sister at play! O, well for the sailor lad, That he sings in his boat on the bay!
The speaker thinks it's all well and good that the fisherman's kid is "shout[ing]" and "play[ing]" with his sister.
Repeating the same sentence structure, the speaker says it's great for the sailor who is "sing[ing]" in his boat.
The repetition makes it sound like maybe the speaker doesn't really think it's all well and good for these people to be cheerful. Is he jealous, perhaps, of their happiness? Or of their ability to communicate it, since he admitted back in Stanza 1 that his "tongue" can't "utter/ the thoughts that arise"?