Legal meaning of seaworthiness has its roots in the law of marine insurance. To avoid liability in case of lost of the ship or the cargo the underwriters often alleged that the ship was unseaworthy at the commencement of the voyage.
In Lyon v Mells(1804) 5 East 428 per Lord Ellenborough CJ at p.436:
In every contract for the carriage of goods between a person holding himself forth as the owner of a lighter or vessel ready to carry goods for hire, and the person putting goods on board or employing his vessel or lighter for that purpose, it is a term of the contract on the part of the carrier or lighterman, implied by law, that his vessel is tight and fit for the purpose or employment for which he offers and holds it forth to the public: it is the very foundation and immediate substratum of the contract that it is so: the law presumes a promise to that effect‘ on the part of the carrier without any actual proof; and every reason of sound policy and public convenience requires it should be so.
Classic definition of seaworthiness was stated in questionable form by Field J in Kopitoff v Wilson and Others (1875-76) L.R. 1 Q.B.D. 377:
Was the vessel at the time of her sailing in a state, as regards the stowing and receiving of these plates, reasonably fit to encounter the ordinary perils that might be expected on a voyage at that season…?
Per Channell J citing Carver on Carriage by Sea in McFadden v Blue Star Line  1 KB 697, at p.607:
To that extent the shipowner, as we have seen, undertakes absolutely that she is fit, and ignorance is no excuse. If the defect existed question to be put is, Would a prudent owner have required that it should be made good before sending his ship to sea had he known of it? If he would, the ship was not seaworthy within meaning of the undertaking.
Per Viscount Sumner in F.C. Bradley & Sons Ltd. v Federal Steam Navigation Co. Ltd., (1927) 27 Ll.L.Rep. 395 at p.396:
In the law of carriage by sea neither seaworthiness nor due diligence is absolute. Both are relative, among other things, to the state of knowledge and the standards prevailing at the material time …
Maxine Footwear Co Ltd v Canadian Government Merchant Marine Ltd  AC 589, per Lord Somervell of Harrow at p.603:
In their Lordships' opinion "before and at the beginning of the voyage" means the period from at least the beginning of the loading until the vessel starts on her voyage. The word "before" cannot in their opinion be read as meaning "at the commencement of the loading." If this had been intended it would have been said. The question when precisely the period begins does not arise in this case, hence the insertion above of the words "at least."
On that view the obligation to exercise due diligence to make the ship seaworthy continued over the whole of the period from the beginning of loading until the ship sank.
In The Muncaster Castle  1 Lloyd’s Rep.57 per Lord Keith of Avonholm at pp.86-87 clarified the scope of the carrier’s duty under article III, rule 1, to exercise due diligence to make the ship seaworthy as below:
[The shipowner] cannot … be liable for unseaworthiness in a ship which results from lack of due diligence at a time when the ship was not his to possess and control and which could not be detected by due diligence after the ship came into his possession… Where due diligence of a carrier is concerned there can be no liability for anterior failure of diligence by a previous owner or by someone with whom the carrier had no previous concern.
The Hague Rules abolished the absolute warranty of seaworthiness. They substituted a lower measure of obligation. The old law no doubt worked hardly on shipowners and charterers, in the absence of exception or exclusion. The change in the law, not confined entirely to England, operated to afford relief to shipowners, as well as some protection to shippers. It would, however, be a most sweeping change if it had the result of providing carriers with a simple escape from their new obligation to exercise due diligence to make a ship seaworthy. … The carrier will have some relief which, weighed in the scales, is not inconsiderable when contrasted with his previous common-law position. He will be protected against latent defects, in the strict sense, in work done on his ship, that is to say, defects not due to any negligent workmanship of repairers or others employed by the repairers and, as I see it, against defects making for unseaworthiness in the ship, however caused, before it became his ship, if these could not be discovered by him, or competent experts employed by him, by the exercise of due diligence.