I've just had a look at the National Archive site, and BT334 is: Registry of Shipping and Seamen: Registers and Indexes of Births, Marriages and Deaths of Passengers and Seamen at Sea. It does say that you can look at images of it online, but I can't work out how - I'm just getting sent round in circles.
I had an ancestor die at sea in about 1920, and had this book out when I was at the Archives - fascinating reading!
The info you need to get a death cert is in the book, but when we got that through it didn't tell us any more than we had found out already from the book.
I'll go and have a look through my computer and see if I have photos of what we saw.
Edit for my awful typing!