Ruth Bloom <ruthpbloom@...>
It might be worth looking for her in the GRO marine deaths register, which you can access online via www.1837online.co.uk (choose 'overseas'). If she's there you can then order her death certificate.
Otherwise I would expect a death at sea to be recorded on the passenger manifest (often a separate section at the end of the list of passengers). There is also, as you say, a list of deaths at sea held at Kew. If you fail to find your relative in the GRO index, contact me privately as I go to Kew fairly frequently and may be able to do a quick look-up for you.
Researching : BLOOM (London, Bristol, Poland)