First off, thank you for making me read a law review article on shipwrecks. That was probably one of the few interesting law review articles I've read. Now on to your question.
Short Answer: No
Long Answer: Still no. The biggest issue is that former colonies trying to assert ownership of a sunken treasure galleon might not have standing in a case, because depending on when the ship sank the former colony was presumably a part of the empire at the time. The New York Times ran a relevant editorial on this issue in 2007.
The United Nations Convention on the Law of the Sea established the International Tribunal for the the Law of the Sea which would be the logical court of record for a dispute of the type you are asking about to be adjudicated. To date, the tribunal has decided 22 cases, and none of those cases involved shipwreck disputes between a colonial master and former colony. The closest thing to your fact pattern would be a case between France and Seychelles, but that concerned a fishing vessel flying the Seychelles flag entering French territorial waters.
As the law review article alludes to, and the NYT editorial expressly advocates, the best bet that former colonies would have to assert ownership over these found shipwrecks would be under a cultural property approach. The United Nations Education Scientific, and Cultural Organization ("UNESCO") actually has a convention geared towards the protection of underwater cultural property. Both Spain and Mexico have ratified the convention, so they likely would never even get in a dispute over a shipwreck. Interestingly, the United Kingdom and the Netherlands are the only colonial powers from that time period that have not signed/ratified the convention, so in the future if a dispute were to arise it would be probably be between one of those countries and one of their former colonies. As an aside, the US has not ratified, nor even signed, either convention.
