I had a Wayfarer insured with N&G a few years back, and I did have to make a claim which proved to be a non-trivial percentage of the boat value. There was no probelm at all - their handling of the whole thing was both helpful and professional. They were fine.
We have a policy with craftinsure who in turn insure with Nav and Gen - we had a significant claim this year and were passed to deal with Nav and Gen - comms sometimes a little bit slow but otherwise really good to deal with, full claim settled without any argument and very supportive throughout
+1 for NIS and N&G. Had 2 claims for a lost propeller (yes i lost it twice!.... don't ask!) . Great service from NIS and N&G both claims were paid out.
I set fire to myself last year. Burnt my musto jacket and sallopettes. Made the crew laugh. N&G paid out in full, no noticeable increase in premium this year either.