A buoy bobs in the water, and the speaker sees something almost human in it: a sad king who stays out in the drink, day and night, with no relief. The comparison is odd but it works. A king ought to have power and a throne, yet this one is stuck in the waves, tethered and tossed about, with a crown of nothing but dignity.
The last two lines are the heart of it. The buoy seems to "mean to lie there" and rest, but "not being able" to. Its very job is to stay afloat, so it can never sink and give up. That turns the image into something close to exhaustion or a kind of weary endurance, a being forced to keep going when it might prefer to stop.
The mood is quiet, tired and a little wry. The poem doesn't state a feeling outright, but judging by the imagery, the speaker sympathizes with this patient, unwilling floater. The title's pun on "buoyed," which usually means lifted up, adds an ironic edge: being held up isn't always a comfort.