220 is a good grit between and up to the final coat. Most in-depth books on varnishing recommend to use a much finer grit prior to the final coat like 400. You will get a better gloss.

Most people just accept their final coat as done and most often it looks great. Sometimes trying to sand and polish out a varnish job can create a lot of work for nothing. I'm not trying to discourage perfection but from 3 ft away the unpolished one and the polished one will look the same. If anything just auto polish on an electric polisher will smooth out a surface. Try a little patch on the bottom before committing to the deck to see if it's worth it. And make sure your varnish is totally dry - like a few weeks. If it's not, you'll never get the gloss back.