180 ruby splashtail

If you have the free hide emote you can just fish at a seapost and hide behind the shop counter of some of them if any players approach. That's what I did when a brig approached and some players with grindy unlocks unloaded some fish and checked the seapost shop. Almost 💩 my pants. Also it helps if you have a tablet to watch YouTube on while fishing for those rubys (and being able to fish mostly without looking at the screen and just going off the vibrations of the controller). Completed mine today. Been fishing non-stop since Monday.

Edit: Sink your ship first.

Just do them at a Seapost, it's pretty easy, just repetitive. Scuttling your ship should avoid most unwanted attention but if someone does come near, you'll have nothing to lose anyway.

Wild Treasures was my choice because there's very little traffic up there (a lot of players hate The Wilds) and both Marauder's Arch and Old Faithful Isle have cannon towers to act as an early warning system. Plus both outposts are visible but face away from the seapost.