A little known fact is that while many cases of flatulence are ignitable, only some people produce methane. Many others release hydrogen in significant quantities.
The hydrogen results when something needs an electron acceptor for some reason, and O2 isn't available somewhere deep where the sun doesn't shine. (Hydrobacteria grow in multiple such environments, some beneath the sea and some much closer to home) If you have electrons to spare in any aqueous environment, you can combine 2H+ + 2e- = H2. (Of course, in air that would be a horrible waste of fuel, but the usefulness of fuel depends on the local atmosphere!)
Now all this depends on some manner of undigested food that needs some sort of what can loosely be categorized as fermentation. Perhaps your sea was on the legendary ancient Bean Trail. Or perhaps more likely, toxic herbivores are dying and scavengers can't touch them.