Findmypast have now been extended to cover the period 1835-1941. This is great news, although not quite as good as it might first appear. This is not the fault of the good people at Findmypast, it is due to the fact that there was no central registry of seamen in the merchant service between 1858 and 1917. This is of course the period that many of us are interested in. There are other sources that you can use for this period, there just isn't a nice 'one-stop shop' kind of service record to be found for those years.
The records added to Findmypast today are those for 1835-1857, which will be of enormous use to many researchers (not including me, as far as I know, but I'm always prepared to be surprised). There is a lot of useful help on the site, in the Knowledge Base area, including a link to The National Archives guide on Merchant Seamen serving up to 1857