Meaning of "while he thereto was lashed" in Byron's Don Juan

Question

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?

Answer 1

In context, lash means be fastened or be bound.

Unravelling the poetic word order, the clause can be rewritten prosaically as:

he clung to it (the oar), and, while he was lash'd thereto, the waters beat sore

Then, paraphrasing and filling in equivalent but more prosaic language:

he clung to it (the oar), and, while he was [thus] bound to it, the waters beat [him] severely

To address your doubt about being “tied to the oar”, he has tied himself to the oar, using his own limbs.