Do you like goldfish? If you don't, leave this thread. And that's how I evade any criticism; intelligence 100.
Have you ever wanted a pet fish, but all the fish you take from the sea don't do anything for you? Well, the goldfish sounds just for you. The goldfish can be found as a decoration in Villages, swimming around small "fishbowls", a result of placing glass panes next to water. This would be a new property of glass panes placed next to water- think of them like trapdoors next to a block of water!
Don't try stealing the goldfish though! Taking a bucket of water and stealing the fish will cause Iron Golems to aggro you. To get a goldfish you have to speak to the fisherman, who will trade you a bucket of water with a goldfish for a few iron ingots or something. It's not the most complex trade but is designed for you to want to come to Villages and interact within their community.
Felt cute might delete later
Goldfish can be placed into a bowl and will start swimming around. They are the smallest fish, only three pixels in length. Once traded for, the fish entity permanently considers you its "master". So if you let it loose in a river, it will follow you around! Alternatively, you can place it in the "fishbowl" structure and it will stop following.
Guardians and Drowned will become distracted by the goldfish and attempt to kill it. The drowned typically fail thanks to the goldfish's agility, while guardians are more effective with their laser. You have to kill the monster before it kills your pet, for else it will turn back to attack you.
So the prime strategy for raiding ocean monuments is to buy all the goldfish from the local fisherman and take them all on a swim to the monument. Most will not survive, but they are loyal creatures that will fight swim to the end.