Convention on the Protection of Underwater Cultural Heritage (2001 November 2)
"For the purposes of this Convention:
1. (a) “Underwater cultural heritage” means all traces of human existence having a cultural, historical or archaeological character which have been partially or totally under water, periodically or continuously, for at least 100 years such as:
(i) sites, structures, buildings, artefacts and human remains, together with their archaeological and natural context;
(ii) vessels, aircraft, other vehicles or any part thereof, their cargo or other contents, together with their archaeological and natural
context; and
(iii) objects of prehistoric character.
(b) Pipelines and cables placed on the seabed shall not be considered as underwater cultural heritage.
(c) Installations other than pipelines and cables, placed on the seabed and still in use, shall not be considered as underwater cultural heritage."
For specific subject searches on SOLO try:
Convention on the Protection of Underwater Cultural Heritage (2001)
Underwater archaeology -- Law and legislation
But don't ignore broader searches such as
Cultural property -- Protection (International law)
Law of the sea
as underwater cultural heritage may be the subject of a chapter within these general works. Some examples of this are below:
The Law of the Sea by 
International Law and the Protection of Cultural Heritage by 
International Cultural Heritage Law by 
Underwater Cultural Heritage and International Law by Various topics of ILOS are considered on other pages on this guide: content includes guidance on books in our collection.