Some sea charms impose minimum sailing level restrictions before you can use them. This charm's restriction is to get a ship from Darby Drydock after the Elizabeth Swann cutscene. Since you managed to loot this I'm going to assume you got at least to that point. Just put the charm in the rightmost slot in the hotbar and it'll give you the listed buffs while you're sailing.
Treasure Sense is a passive sailing skill that gives the pirate the ability to locate and haul in more treasure from the ships they plunder. Treasure Sense is unlocked at Sailing skill - Level 14
Open Fire is a Sailing skill that temporarily increases all Cannon and Broadside damage by 50% for a few seconds as the captain coordinates his/her crew's guns. This skill is unlocked at SailingSkill - Level 10. When the captain calls it out, cannons fired will have glowing cannonballs to indicate the skill is in use.