@trissiboy462
I am fairly sure it's a Rare encounter.
I have seen two so far, one when first encountering the Two factions of the area and another randomly when exploring the islands around the place.
So i'd say it's rare but it's not a "one time done" type of deal.

Can you only encounter the Tankker during a story mission and then never again? I haven't obtained the achievement for boarding it yet

There are also 3 secrets ships only by boarding
The Tankker,destroyer and speedboat

guide needs updated as some of the gun placements on some ships have changed. Example: the cutters rear guns were moved to the forward position.

first Shipshape i found in SeaStart Cove. Icon looks line the bundle of wood

Where are the shipshape shops?!?!? I've been looking for 1 hour

nice to see the available ships!
great upgrade to this guide would be to also add how to aquire them.
still, here's a prize