I've varnished my boat's deck and hull at once in two different ways. The first way is to set the boat upright on sawhorses and start at one end and varnish to the other, in about 12" bands, maintaining a wet wedge. Then the next day I touch up the two patches on the bottom where the sawhorses were.

For the most recent varnish job, I went to the trouble of suspending the boat upright using supports inserted in the hatch openings. This allowed doing the whole job seamlessly.

Doing the hull and deck at once means you really have to work fast and careful to maintain a wet edge, and means you have to work from underneath as well as above, but I like the convenience of getting it all done at once.