Stamina in Sea of Conquest is consumed while battling Armed Freighters and Merchant Ships. You can increase your stamina limit by clearing Grand Odyssey missions, and certain tips will help you use your stamina efficiently.
Below, I’ll be giving a short overview of stamina, and then we’ll see what’s the best way to utilize it.
What is Stamina?

Stamina is basically the energy consumed to fight other ships in Sea of Conquest. It is represented by a curved green line under your avatar.
Besides, if you click on your avatar, you’ll see that the maximum stamina limit is 1500. Moreover, it will take 60 seconds for one stamina point to fill up again.
TIP: Never leave your stamina at full capacity if you can help it. Not doing this wastes all the stamina you could've recovered had they been spent, as well as all the resources and Adventure EXP you could've looted from enemy ships with all the extra stamina!
How to Get Stamina in Sea of Conquest?

You can increase the limit of your stamina by progressing in your Grand Odyssey. missions.
Clearing Grand Odyssey Mission 19, for example, adds 1500 stamina to your base limit, giving you 3000 max stamina instead. This allows you to sink more ships for Adventure EXP and helps you get stronger fast.
Additionally, you can attack Armed Freighters and Patrols even without stamina!
The Best Ways to Use Stamina
If you want to get the most amount of Adventure EXP and loot for every point of stamina you consume, then make sure to heed the things I'll mention below.
Search For Other Ships And Attack Them

Fighting Armed Freighters and Merchant ships costs 60 stamina each. So if I have 1500/1500 stamina and I sink an Armed Freighter, I’ll have 1440/1500 left.
Fighting either ship of any level costs 60 stamina.
Looking at that, you should always focus on battling ships of higher levels in Sea of Conquest to get premium loot and more Adventure EXP. In fact, battling high-level ships is one of the best ways to level up fast in Sea of Conquest!
To make it more clear, if you fight a level 2 Armed Freighter or a level 20, both will cost 60 stamina. However, the difference in the loot given (especially Adventure EXP) by both will be huge.
Therefore, always use your stamina to fight ships of higher levels.
TIP: If you want to get even more loot, then you should call on your fellow Gang members for help in taking down much higher-level ships that you otherwise wouldn't be able to sink alone!

Also, if you want more convenience, then you should battle Armed Freighters more often than Merchant ships. The reason is that you can select the level of the former but not the latter.
Merchant ships are located in a certain region, and they can be of any level. In short, you can’t spawn them like Armed Freighters.
Also, they have no fixed location and roam all over the map, so you might have to give chase if you want to attack one.
Whenever you see lower-level Merchant ships in Sea of Conquest, remember that there’s no use wasting your stamina on them. Instead, spawn a high-level Armed Freighter and take it down!

If you do happen to see a high-level Merchant Ship that you can take down near you, then feel free to attack it.
After all, they give the same amount of Adventure EXP as Armed Freighters of the same level and drop significantly more goods that you can sell for Gold, as you can see in the image above.
WARNING: Attacking Merchant Ships increases your Infamy. This gives you a Gold bounty that may incentivize other players to attack you, so be careful and don't sink too many Merchant Ships!
Watch Out For Leader Ships

Leader Ships (a.k.a. patrols) randomly appear around an area where several Armed Freighters have been sunk—as you can see in the image above.
If you’ve been hunting too many Armed Freighters yourself, then you might just encounter one of them!
Don’t worry, they’re not much stronger than regular Armed Freighters, but unlike them, Leader Ships cost 300 stamina to attack.

This may seem like a lot, but you actually get a pretty decent ROS (return-on-stamina) if you attack Leader Ships because they give approximately the same amount of loot—Adventure EXP included—as five Armed Freighters of the same level do.
So if you were planning on sinking five more Armed Freighters anyway, then go ahead and sink any high-level patrols you can find instead as long as it has a similar or higher level.
Conclusion
Since you can’t get stamina in Sea of Conquest, you should always look to spend it to farm as much loot as possible.
Always aim for the higher-level Armed Freighters to get better rewards.