The Oxford English Dictionary says:
heave of the sea: the force exerted by the swell of the sea in quickening, retarding, or altering a vessel’s course.
So the ship was heaved by the sea and this caused the woman to fall, perhaps when she was walking along the gangplank to or from the jetty.
What have the moon and the sun to do with it? These celestial bodies are responsible for the tide, not for the swell, which is caused by wind. The tide does not move fast enough to noticeably heave a ship (except in a few places where there are tidal bores or races). Perhaps Hughes is making a metaphor whereby the sun and the moon cause the tide, and the tide is like the swell, and the swell moves the ship. However, the chain of connections there seems rather long.
In the next line, “humbled out” is also hard to interpret. Turning again to the OED I find that ‘humble’ has a second meaning:
humble, v.2 Obsolete. intransitive. To rumble; to mumble; to hum or buzz as a bee.
so perhaps Hughes means that her cries were drowned out by the noise of the ship’s engine.