when they get down to three or four people will they be able to sail a ship that size?

Lucky for the cast, there is a whole crew of real sailors sailing the ship. The "Ghost" Crew stays out of view but are the real captains of the ship.
So even when we're left with 6 or less piratewannabes, the ship will continue to stay afloat. (Although, knowing Burnett, he may have some sort of Island Marooning planned for the final5 or something. That way they don't have to explain the lack of crew available to sail a ship of it's size. Due to the new land based "twist".)

As far as the intricasies of the shows weekly layout, I have no hard or fast answers, yet within some of the current preview video captures for next weeks show, it looks like everyone starts out on the same level for the next Treasure Hunt segment.
I'm guessing that whichever team finds the 2nd treasure will again vote for a new captain. (Or keep the old one if they choose.) And the rest of it plays out like it did the first week. 2 Officers, 3 BlackMarks, 1 Marooning.

But my record on correctly predicting these things is notriously lacking any noticable checkmarks.

I'd provide the link to the "spoiler" viccaps but it feels wrong to do so in a clearly fanatics/bashers topic.
But if you want to see the viccaps in question you can find it by looking for ScarlettOHara's Ep.2 Spoiler Thread.
In this Forum.