After playing Bloodstained: Ritual of the Night for many hours, chances are you’ll find yourself at an impasse. At that point, it’s likely that you need to explore underwater.
There are two steps to being able to walk underwater in Bloodstained: Ritual of the Night. First you need to obtain the Aqua Stream shard which allows you to travel underwater by firing out a jet stream of water. It’s tricky to use, however, and doesn’t allow you to perform certain actions underwater such as opening chests or using other abilities. To obtain the Aqua Stream shard, kill the Deeseama enemies found in the Forbidden Underground Waterway. They look a bit like jellyfish.
Once you have the Aqua Stream shard, you should be able to explore underwater. Do that, and after getting past a load of spike traps you’ll eventually find a new area called Hidden Desert. Beat the boss there and you’ll obtain the Deep Sinker shard, which allows you to walk underwater, albeit very slowly.
With the Deep Sinker shard you’re able to open chests underwater and use other shards such as Reflector Ray. That enables you to further explore underwater and eventually discover the Inferno Cave, which is important if you want to obtain Bloodstained: Ritual of the Night‘s true ending.