Maybe not directly piracy but being part of different navies as commanding officer might work. You might have to keep faction relations (like in GTA 2) and handle missions and piracy might be like going rogue and have your own country but something really hard (like Mount&Blade expansion mod)

Well, semi-realistic... do you want "pirates in WW2 set on earth without any alternate reality crap"? Or could you also be satisfied with a "pirates in an alternate universe / alternate reality, with WW2 style technology, and a world war going on in the background"?

In case of the second, well, the skys the limit really... but to add to that, pirates often started their pirate life as a kind of "guerilla warship / sailors" of one of the big navies in the 17th/18th century. While all of them did what they did mostly to fill their own coffers, many did it with the backing of a big naval nation in the background. Often they would pay a part of their bounty for this backing. Some would not and become real pirates. Some would become national heros on the other hand.

This was often done in "peacetime" in order to damage the economy of rivalling nations while filling your own nations coffers at the same time. But these freebooters would often also partake in naval warfare during a war, either continuing their raids on rivalling nations trade shipping, or sometimes even joining the ordinary naval forces during their battles.

So piracy has been a part of naval warfare for a long time. Hell, if you look at naval warfare in WW2, most of it was aimed at trade shipping and the ships delivering the lend-lease program goods, or the prevention of such raids. There was almost no open naval battles anymore like in WW1, because nobody was ready to pitch their expensive capital ships in a deceisive battle.

If you overlook the fact that most strikes on trade shipping was done in a way that just sunk the victim ship, with the attacker trying to get away as fast as possible instead of trying to pilfer anything from the victim ship, most naval action in WW2 where going in the direction of piracy.

Now, if in an alternate reality there was no total war, thus the nations involved in the war wouldn't have tried to win by any means possible in an inhuman battle of attrition, and thus "pirate ships" would have the time to pilfer enemy trade ships without risking the whole grand fleet being sent out to get them afterwards (which, given there are enough pirates active, wouldn't be an option anyway... there are only so many battleships and battlecruisers a nation can afford to have active at the same time, and the cruisers alone the pirates probably could deal with), if the nations would have instead of trying to end the war as quickly as possible (this was most probably one reason for the whole "total war" thing) tried to just win by getting the economical upper hand over the other nations, if needed by pilfering their trade shipping, piracy in a WW2 setting would be a very realistic possibility.

Maybe take your inspiration from the 17th/18th century time when piracy was on its peak, and merge those influences with WW2. That might give you a scenario where WW2 technology and the general world order during that time could feasibly merge with the pirate setting.

Buying ships is a little odd, not impossible. The French have built warships for other nations, even recently. It's a slight stretch to sell that to a third party, but in a dystopian world where mercenary/pirate companies are the norm, I could see it.

Stealing and salvaging ships isn't too bad. You can control the salvage by making it only a small percentage chance of a ship that was attacked ends up being salvageable, or force the player to launch boarding parties to take the ship, etc. The Battletech games would often do the former, while space games would often do the latter.

Stealing ships from ports is also highly plausible, it's definitely been done.

And I could definitely see a nation giving away it's ships, like all those 'pirates' who were really under the employ of the various sea faring nations back in the days of sailing ships.

And I could see an advanced pirate nation/base possibly building it's own ships, but that's probably the biggest stretch. Modifying existing ships into warships would be more likely. Though they would obviously be inferior in every way to a real warship, when fighting against mostly unarmed or lightly armed/escorted trading ships, they could get the job done.

And of course fuel concerns make pirating in a battleship completely unfeasible, but I at least wouldn't care, I doubt any grognard who is okay with an alternate reality would be too put off by it.

I wrote mercenary/privateer in my first post :P (so not ONLY traditional piracy).

The world is sligthly post-apocalypse, to allow for a more open political scene. It's a "total war" scenario where the big nations switch alliances etc. So total realism is out the window.

You can buy ships from big nations when you are trusted enough (you gain ranks with them by running missions from them)

You can buy ships from independant city states (since they want you to be a counter to the nations, which seek to conquer independant city states).

There is "havens" which always sells ships as well.

Good input, ya'all!

Oh, that makes it easier.

Lore wise, pirates could have never been wiped out/ gained skilled manufacturing and it could be that pirates actually sell their own ships to the major nations as well, so major nations would be willing to sell their own ships, if the cost could cover a better pirate ship.

You could also have a corporation / nation state / guild thing. Maybe some form of the Dutch East India Trading Company. Strong enough to make or buy/sell ships outside of any particular nation.