The speaker is on holiday, and the title hints at what they most want from it: a "desideratum" is something longed for. Yet the poem opens with a small tension. They have "instructions," some sense of what they are supposed to do, but these feel empty without the sea. Everything depends on being near it.
The middle lines change the mood. The speaker gives the sea their "full gaze," and the sea becomes a person, with a "conversation" of its own. Watching the water turns into a kind of listening, as if the waves were talking and the speaker were content simply to pay attention.
The ending adds a gentle joke. The instructions are "as general / as the rain," so vague and widespread that they hardly instruct at all. The speaker seems to be letting go of duties and plans. The mood is calm, relaxed and a little playful, a short moment of being absorbed in something larger than any to-do list.