Could someone please explain the game to me?
Sandbox Pirate MMO.
Yes, but it's not a cinematic over the shoulder single-player narrative with big set piece cut scenes. The storyline is told through Tall Tales and over seasons of content. There are also Pirates of the Caribbean and Monkey Island Tall Tales.
What kinds of quests are there?
Each trading company has different types of voyages. Merchants are finding or delivering cargo, Gold Hoarders are digging up treasure or unlocking treasure vaults, Order of Souls are high-seas Ghostbusters hunting skeleton captains for their skulls. Then there are sea forts and skeleton forts, ship fleet battles, and a whole lot of other voyages. There's never not something to do.
Do you just wander around aimlessly and search for PoIs to loot?
You can or you can do a voyage which is basically a quest.
You can level 1-100 with each trading company. Once you hit level 100 you unlock a cosmetic ring and then can reset your progress back to level 1 and level up to 100 again (like CoD Prestige used to work) for another ring. Once you hit level 50 with three of the trading companies you unlock the endgame Athena's Fortune trading company for Pirate Legends that you can also level 1-100 in. There are also two PVP specific factions for the opt-in PVP (Hourglass) mode. Reaching max level in each unlock a specific curse that changes your appearance (Ether a Ghost or Skeleton).
Does additional game content cost money?
No. There's a seasonal battle pass with a free tier and a paid tier that unlocks cosmetics. The battle pass is $10 for the paid tier, it gets you that much back in Ancient Coins (the shop currency) and it unlocks a ship set that will later sell in the shop for about $40.
What is the gameplay loop like?
It's whatever you want it to be.