Theft requires not only the intention permanently to deprive (UK law) but also "dishonesty". "Dishonesty" is a very murky pool indeed, but with the knowledge that Royston has provided above, any of us who decided that a bit of romantic wrecking or scavenging looked fun would indeed be very likely to be guilty of theft.

I speculate however that if cargo was deliberately jettisoned to save a ship, that mght constitute "abandonment", resulting in a different legal position. The argument has been successful in the case of material in a skip or abandoned on common land - but marine law is a funny animal.