You mean interior
I almost bought a boat with damage on the interior
from left open hatches. Could not find any satisfactory way to cure it other than replacing the panels. Cutting back the varnish
and refinishing did not help (we experimented with a couple of spots) because the wood was blackened by the soaked-in water
. We also tried painting over them and imitating the grain -- a highly skilled cabinet maker did this. Looked like complete cr*p. Sorry if that is unhelpful, but it was my experience.
If you have white
streaks, on the other hand, like coffee cup rings on a table -- this can be solved
with a piece of cloth and an iron, like for ironing clothes.