I have about 15 years experience with my current boat. That includes revarnishing everything below at least once. And lot of experience with teak cleaner on a previous boat. It is hard to get both good looking and non-slippery floor boards. My solution has been throw rugs with rubberized non-slip backing. I do recommend using exterior varnish even below decks on a boat. The cost savings of interior varnish are eaten up by having to do it again sooner because of the fading.