From Byron's Don Juan:
Nor yet had he arrived but for the oar,
Which, providentially for him, was wash'd
Just as his feeble arms could strike no more,
And the hard wave o'erwhelm'd him as 't was dash'd
Within his grasp; he clung to it, and sore
The waters beat while he thereto was lash'd;
At last, with swimming, wading, scrambling, he
Roll'd on the beach, half-senseless, from the sea:
I don't understand this. The oar is by luck carried by the waves towards him. He takes the oar and "beats the sore waters" using this oar. But what is the meaning of thereto was lashed? Was he all of a sudden "tied to the oar"? That's unlikely.
Or does lash means "to beat, to strike" here? In this case, I cannot understand the overall meaning of the phrase. "He was beaten to.." -- to what?