You have limited supplies on the open ocean, and your main goal is to get treasure. Every fight with other ships drains your supplies- wood planks for repair, food for healing, as well as your cannonballs. Treasure chests are carried aboard your ship and while having more of them doesn't slow you down as per encumbrance, it does make for more risk and more reward- as its a gamble to keep sailing when you already have a bunch of chests as you could be sunk and lose it all. Some treasure chests are also cursed and add to more resources- such as the Chest of Sorrows which cries and fills up your ship with water that you have to bail out.
While the open ocean is hostile and unforgiving with many encounters; skeleton crews, other players, sea monsters, storms and so on- islands are your sources of respite. In many ways these are like towns, but islands are even better in D&D towns because while they can be used for resupply they can ALSO be plundered for treasure, some of them being inhabited by dangerous skeletons or traps as well. Plus solving the riddles from each treasure map to find treasure is just genius. You could totally port this game right over into dungeon crawling adventures with or without the basic classes or typical rules and it would feel right at home.
12 Random Pirate Treasures – Roll to see what you find in a pirate's treasure chest!
 Doubloons matey!
 Mummified Monkey. Worth a fortune to one insane collector.
 Turtle Shell Hat. Makes you look like a turtle while you're swimming, +1 AC
 Crate of limes; protects against scurvy.
 Jollyjack Sword. The OG pirate's cutlass. +1 magic weapon, cuts your flag into people's skin.
 Mermaids comb. Valuable on its own; present to the mermaids and one will marry you.
 Walrus cream. Rub on skin for good ointment- makes you smell nice. If you put some on and smell like it around a walrus; it will want to marry you. Right now. On the beach.
 Salted Megaladon Meat. Very tasty, restores 1d6 hit points for being such a delicacy.
 Reed Whistle of Winds. Blow the whistle and the winds blow as long as you can hold your note.
 Rat stick. It's just a stick used to beat up stowaway rats. Deals 1d2 damage. Somehow every rat knows about this stick and fears it. Rats make a morale check when hit.
 Bastard Banana. Looks like a banana. When unpeeled, unsuspecting fruit-eater is attacked by the hundred venomous spiders just released. Pick up the peel and fold it up to reuse.
 Cursed Cannonball. +2 to hit. If it lands on the enemy ship, will lodge and plug it's own hole in their hull; appears ineffective. Actually curses the whole ship as long as its onboard.