Knew I'd forgotten something. The ship doesn't present a considerable problem in ownership, I agree in that regard. I should've named it, different from Vultiph, as 'The Ship that Theseus Sailed in On' firstly, and then Theseus' ship thereafter, because we wish to identify the ship later as identical to the ship before. If we only wish to know which ship belongs to Theseus, the problem isn't as large, and I believe your reasoning that meaning is use is correct.
I'm not arguing it is a problem of ownership. I'm arguing its a problem of definition. Perhaps that isn't clear. No matter what you call it, if we agree it is the same ship, then it is (by definition), no matter how many parts have been replaced or who currently holds the title. If not, then it is a different ship. That is my basic point.
So, you're saying that Theseus' Ship and The Ship Theseus Sailed In On are the same ship if they refer to the same object? That is to say, if we both have the same object in mind when we talk about them? I'd really like a more specific way of identifying if they are the same thing; how would we explain to someone who hadn't seen either ship the features that make them identical? Or would you say they'd accept them as identical the second we claimed them so through our use of language?
How would you explain a jury trial to someone who had never observed one? Descriptivism doesn't work, because there are more levels of understanding than an atomized one (and besides, the putative words you would use to describe a higher level concept still need definitions themselves). Thesues' ship is Theseus' ship, because we say it is. The boards from which it is made are irrelevant. There is no baser level of explaining than pointing at an object an naming it. This is how we learn language.