Step 1:  Skim the "Table of Contents" and indexes of reference books. This will give you an overview of what topics might fall within the realm of law of the sea. 

Step 2: Try to get a feel for what topics are hot right now. Look at websites of organizations, departments, or government entities involved in law of the sea issues. Do searches for relevant news articles and sources. Don't forget to skim blogs!

Step 3:  Jot down some topics. Look at what has been published in recent journal articles. Eliminate any topics which have been done to death.