Is this performed as a possible action or is it a one off

It has an action header, meaning it's an action. You can use it whenever you want as long as you can perform an action.

That said, Marksmanship is pretty much the biggest trap in the game. It's almost never worth it. Ships that can make multiple attacks in one turn are the only ones I would even consider it on.

TO elaborate:

You can do the Marksmanship action whenever you can do an action - ytypically in your Perform Action step.

Then, every attack you make for the rest of the round, it will take effect and turn all your eyes to hits and 1 to a crit.

It is an upgrade that requires an Action to activate for the round and sticks around to be activated again in later rounds.

Once it has been activated Marksmanship remains "on" for the remainder of the round and will modify all attacks the ship makes during that round.

Also to elaborate more if it was single use it would include a phrase saying to discard after use. The closest thing I can think of to being a single use action is Burnout SLAM.

Just a tip, it has good synergy with Bossk.