I once had the argument with another insurer "So if I leave a $20K bicycle unsecured and it is stolen you will cover it..... if a surfboard is stolen you will cover it....... but not a $2K windsurf board in a locked car as you say it is actually a boat?"
Reply: "umm, yes"
Needless to say I left them
Excluding car insurance, I think I've made a total of 4 insurance claims in my life (incl travel, health etc), and 3 of them were initially rejected. I disputed all 3, threatened to take it to court on 2 of the occasions, and ended up getting each of these claims approved in full or in part. Insurance companies try on all sorts of rubbish arguments that dont stand up to scrutiny once you actually read the policy. One tried to argue that the common area of a shared house was "public space" so not covered against theft. Similarly, I'd like to see an insurer explaining to a court why a windsurfer board should be considered a boat.
Moral of the story: don't be shy to push back. Most of them will baulk at the idea of having to send someone to appear in the Magistrates Court over a $1000 claim.