I've finally managed to find my great great grandfather's death. I found a marine deaths index in my local library and found that Henry Alexander Anderson aged 48 died on board the Nentwater in 1883.
Now my confusion comes with the fact that this index does not include a page or volume number as you get with the regular BMD indices, so will there be a certificate for this death?
According to his daughter and wife's marriage and death registers he was a master mariner and I've found from browsing on the internet that the Nentwater did not sink or have any accident in 1883, as it was around until it was scrapped in 1931.

Thanks for any replies.

Katrina