When I erase the tracks on my Raymarine GPS, is the data really
destroyed? Or can it be recovered like the deleted files on a computer
hard drive?

I'm sure someone in the gov't could well extract the chip and reconstruct
what had been written to it. But if you're just talking about "will my
wife/parents/insurance company/police be able to easily recover it" the
answer's probably no. Bear in mind, however, lying about only ends up
making it worse, doubly so if you file something fraudulently, triply so if
the wife figures it out.