NT wrote:

It seems Lithofin and HG are both volatile solvent based, ruling them
out. Will look into yacht varnish and experiment. Cheers,

Errm yacht varnish is solvent based. Indeed all paint/varnish is based
on a solvent even if that solvent should happen to be water. But the
solvent used in yacht varnish is a volatile organic solvent which I
suspect is what you're trying to avoid (I have no idea why, other than
middle-class trendiness/desire to **** about and troll).

Yacht varnishing ceramic tiles could be about the most stupid trick
pulled by anyone.

The rubbish sold in DIY sheds as "yacht varnish" isn't suitable for
continuous immersion. It's not even proper yacht varnish. If you want
something that can be immersed continuously then you need a two-pack
polyurethane varnish you'll also have to thoroughly degrease the surface
that you are varnishing, probably with acetone a volatile organic
solvent, and even then I don't hold out much hope of it working.