That's almost as loaded a question as how many engines are best.
Boat manufactures like American Marine/Grand Banks used many coats of varnish.
The crew of a corporate yacht I was associated with for awhile used a waxoil product from New Zealand that did a wonderful job of masking scratches (from guest's shoes) hard to see, was a beautiful finish, and was easy to touch up. It's initial application was somewhat exacting but we were considering this for when we get around to refinishing the parquet sole in our boat. We have since decided to do something else.
Floor finishes is a long-running topic of discussion on the Grand Banks forum with several excellent processes described in detail from the likes of Bob Lowe and others,
Grand Banks Owner's Resources. Put "refinishing cabin sole" or some such in the archives search and read away.
There are also some good water-based polyurethane finishes on the market now. As a result of a multi-year "experiment" with the inside step to our main cabin we will most likely use one of these, Daly's SeaFin Aquaspar satin finish, on our cabin sole when we get round to refinishing it.
Seven or eight coats of it has held up amazingly well since we applied it to the inside step through the main cabin door some four or five years ago. Everyone steps on it, including the dog, every time they enter or leave the boat and the step today looks pretty much exactly like it did when we installed after re-finhishing with the Dalys product. So we're pretty much sold on it.
But there are lots of options. The factory varnish finish on our parquet sole has held up amazingly well for the last 40 years. In fact it's hard to believe it's the original finish, but it is. Not that it won't benefit from a refinishing but that's more to get out the dings and gouges rather than replace a failing finish. Wherever the parquet has not been subject to things being dropped or dragged on it the original varnish finish is in excellent shape. The process and product that was used back then by the factory is described in the archives of the GB forum, probably several times.
But I think we'll stick with our Dalys' plan if for no other reason than we've been so impressed with its performance on our step.