PirateBattle.io is all about taking to the seven seas (or one decently-sized sea, technically) and blasting any other pirate that dares glide past your bow. Along the way, collect coins to upgrade the different stats on your ship (speed, health, damage, armor). Enemy kills spill gold all over the water.
Steer your ship with the mouse, and press the left mouse button to shoot. Pressing space will give you a short speed boost.
As you collect gold, you'll be able to upgrade your various ship stats by pressing 1, 2, 3, & 4. They will have a plus next to them to know when you're able to upgrade.
Your health (and the health of any opponent) is shown by the state of your ship. The closer it looks to falling apart, the lower your health will be.
The larger ships can take a quite a few cannons to destroy, so unless you view yourself as a superb pirate, you'll probably want to avoid the big dogs until you yourself are a big dog. But if you see any ships on the verge of being destroyed, try to slip in and soak up some of the gold that drops before speeding away with a speed boost.