Jim K wrote:

On boats the sandable caulk used is Sikaflex, particularly for caulking
decks and below the waterline. I'm not sure if this is what you have in
mind.

is that the polyurethane based impossibly sticky (til "dried") stuff?

Yes, that's it.

used something similar to seal frame joints on our hardwood
conservatory - very impressive too..

It is, and once it has cured it can be sanded just like wood. It's used
to make fake teak and holly inlay decks or simply to caulk joints in
decks. Once sanded it's difficult to tell that it is a caulk.