For me there are only two versions of Jess. Our Jess and mean Jess. However there are always three of them onboard at any one time.
Our Jess ends up throwing her same version overboard. At that point, the boat arrives with soon-to-be mean Jess, whereas another mean Jess has been onboard when our Jess first arrives on the ship, but isn't seen until our Jess shoots her in the head. Before that scene, our Jess points a gun at recently arrived Jess, who is only seen one other time after that - hacking gunshot wound Jess to death and dumping her overboard.
Our Jess then hides the bodies left by gunshot wound Jess, then puts on the mask and becomes the same version of Jess we originally see being thrown overboard.
Our Jess always pushes another virtually identical Jess overboard.
Mean Jess always gets a gun pointed at her by our Jess (whichever version), and always kills another version of mean Jess.
For me there are only the two time loops - one where she doesn't discover that there are other versions of herself until she sees the group on the yacht, and the Jess that has the gun pointed at her and becomes aware/mean Jess.
These two loops overlap to form a triangle, with three versions (either two 'good' Jess's and one mean Jess, or one good Jess and two mean Jess's) always on the ship at any one moment. There's no way out of the loop because the two timelines overlap in such a way that she either intervenes because she doesn't know any better, or where she doesn't intervene because she's already had a gun pointed at her by our Jess. The actions of both cause the triangular loop, as her good and bad sides of her character are forced to battle for eternity.

Also it looks liek only the jess we follow (or that loop) is the one that makes it out. So only 1 in three loops get;s to the driving part. Unless a jess that is hacked to death can awaken on the beach.
Although I don't really see that as important: as the ending segenst she is in the field of punishment: THis is no loop ior anything that matters, she is being punished in the most horrible way to herself. Sher will always be killed by herself. Either by axe or car.

Actually it's both a loop
and
an overlap. But each new Jess is definitely not a new iteration. Each Looper Jess we see are all quite literally the same Jess (meaning not a new one driving to the harbor, therefor starting a new overlapping loop chain). Once she starts looping the same Jess loops around at least over 60 times contiguously. I've documented what we observe in an Event Matrix that can be download from here (the first worksheet depicts what we actually observe in the film):
http://sdrv.ms/MoXetr
With extensive exposition here:
http://www.imdb.com/board/11187064/board/thread/223982092

I'm something new entirely. With my own set of rules. I'm Dexter. Boo.

Yeah but the problem is that it isn't a loop, it's more like a constant overlap. Each New Jess is a new Jess and is altering the fate of things. This will happen constantly until it all breaks down.

Close. Very close. Except there are always three Jess versions. Jess Prime meets the last two Jess's in the loop before they exit or die, i.e. when the loop ends. The nature of any time loop is that the end feeds back into the beginning. So in the final loop Jess must do something other than kill her original self at the house, which allows her to drive to the harbor. I go into it in detail in certain sections here if you're interested:
http://www.imdb.com/board/11187064/board/thread/223982092?d=223982092#223982092

I'm something new entirely. With my own set of rules. I'm Dexter. Boo.