My understanding is basically that it's up to what local laws allow, combined with ship owners policies.

We saw what was most likely pirates a few times, but we had armed escorts so they were never openly hostile. You can't just shoot at any that vaguely resembles pirates, as there are fishing boats in the area as well, and they don't look that different.

Story time!

I remember chilling on the back deck, doing my hour of lookout in case someone were trying to board us. One of our chase boats (armed escorts) contacted me via the portable VHF I was carrying to let me know there was a small boat looking like it was trying to approach. I found this odd, as the only boats I saw in the vicinity were boats that were supposed to be there. I looked over the railing, and knew what the chase boat was talking about: Yup, there was a boat there, alright. But it was one of ours, they were just normally much further away. Turned out they were almost out of water after being in an open boat for an entire day that was abnormally hot even by Nigerian standards. I gave them a tray full of ice cold coke cans and a few bags of doritos from the workshop next to me, and they grinningly went back to their job of towing away fishing nets that had drifted into our survey area.