There were no
central records of British merchant seamen (apart from Masters, Mates and some Engineers) kept between 1857 and effectively 1919. Records for WW1 were kept but for some reason these have been destroyed. Researching mariners from this period is always difficult.
Just a couple of explanations to some of the posts.
Seamans Pouches ( they are stored in paper pouch's at Kew, they were not a worn or carried) The documents contained within, except if the individual had service from the Fourth Register of Merchant Seamen, are, by and large post
1941. For details see.
http://www.rootschat.com/forum/index.php/topic,519246.15.htmlLloyd's Captains Registers are indexed for the letter "F" but John Frimstone is not listed. This could be for a number of reasons most common of which, he was engaged in and certificated for the Home Trade or he was not a Master.
Memorial University of Newfoundland records of Crew Lists or more correctly Crew Agreements are indexed by ships official number and cannot be searched by an individuals name. Note that some Crew Agreements are stored in County Record offices, Kew, and NMM
I think the first place you should look is in BT154/7 at Kew and as already stated this would mean a visit. Be aware that seafaring by its very nature is an occupation not easily regulated or documented so be prepared for a long haul.
I have found a John Frimstone born 1860 in Flint North Wales on board the vessel NORA CAINE o/n 62972. Too young for your man but maybe a relative?