For the UK the ONLY hours that really qualify for Certificates are those under UK Certified skipper
. For general paid crew work then UK certificates, and maybe the USA version (no-one respects others as much as their own), will 'qualify' you for better paid positions, or less fees
to travel with them. Unfortunately most of the crewing jobs seem to involve you paying to enjoy the experience. You'll need a good reputation and worthwhile TRACEABLE references
to be paid. There is decent pay rates on 'Super Yachts' but they need info as above.
For an informal log you will also want to get your passport and or log stamped to confirm the destinations you have travelled to, and signed with note of special skills (a reference in effect), again by a traceable Captain
or Officer and his Certificate Type and Number. His Certification
Authority will be able to confirm that he does exist, your passport stamps will confirm that you did indeed arrive at these places.
Be aware also that many countries will want you to prove you have independent means and can afford a normal airline ticket back to your home country.
If it all works out it's a great life.