Otto

Collect Copper Coin of the Isles through fishing or by spear fishing Lunker Threshers using the Iskaaran Harpoon. Trade 15 Copper Coin of the Isles per Silver Coin of the Isles, then trade 5 Silver Coin of the Isles for 1 Gold Coin of the Isles — requiring 75 Copper Coin of the Isles in total unless you find higher-value coins while spear fishing.

Bring your Gold Coin of the Isles to The Great Swog, found in a cave in Ohn'ahran Plains at 82, 73, and purchase the Immaculate Sac of Swog Treasures. The sac has roughly a 60% chance to contain Aquatic Shades — if you do not receive the Shades, repeat Steps 1 and 2 until you do.

Equip the Aquatic Shades and travel to the Waking Shores dive bar at 19, 36. Stand on the dance pad next to the bar at 19.62, 36.5 and dance for five minutes. You will be teleported to Vazallia's hangout spot, where the Empty Fish Barrel will appear in front of you as a usable object.

Fish 100 Frigid Floe Fish from the open waters around Iskaara in The Azure Span at 13.7, 48.5, then use them to combine with the Empty Fish Barrel to create the Half-Filled Fish Barrel.

Fish 25 Calamitous Carp from the open lava around the Obsidian Citadel — try spots at 37, 69, at 33, 64, or at 27.95, 53.53 in The Azure Span. No special Tuskarr reputation is required to fish lava here. Combine the Carp with your Half-Filled Fish Barrel to progress it further.

Fish up 1 Kingfin, the Wise Whiskerfish at 55.93, 44.64 in Thaldraszus and combine it with your barrel to complete it, then interact with Otto when the quest prompt appears — click Otto promptly to complete the quest before he disappears, or you will need to re-fish the Frigid Floe Fish, Calamitous Carp, and Kingfin again.