While the size and shape of different ocean ruins vary a lot, they have some things largely in common. Most of them contain chests packed with tasty loot, including enchanted books, emeralds, buried treasure maps, and enchanted fishing rods. Also, they’ll almost always be buried under a lot of suspicious sand or gravel, making them a great place to hone your archaeology skills.
Be wary though – these ruins are guarded by their former inhabitants, underwater zombies known as drowned. If you approach, they’ll stir to life to attack you. Many will attack with their bare hands, like regular zombies, but others come armed with tridents that they’ll hurl in your direction as you approach (or flee). Be warned!
Finally, a quick tip – if you want an easy way to get down to a ruin without using too much breath, then look for one that has magma blocks surrounding it. The bubble columns that these blocks produce underwater will allow you to descend fast. Getting up to the surface again, though? That might be more tricky.