EPQ Title HelpWatch
The first thing I thought of was investigating the feasibility of mining the metals and minerals produced by underwater hydrothermal vents, for example. You can also incorporate several topics of interest into the essay under that title, such as the chemistry of the surrounding sediment like you mentioned, which not only allows you to research more of the topics that interest you, but will also help you to meet the word count criteria if your title question is a bit too narrow (although you can also edit your title of course!).
I hope that helps