Level 50 Hunter's Call. This takes a while.
Really focus on cooking your caught fish (turn them brown-ish) and turn in any Mermaid Gems that you find. This speeds it a bit up.
Also check wrecks, for in the barrels you can find all kinds of (rare) fish as well.
Use the monthly free level up with Doubloons from Duke/Jim.
It takes time, so just keep on turning all these in and you can buy your upgrades from the faction itself and reach 50.