This page of the Returnal game guide answers whether you can swim or dive in the game. This is an important matter when it comes to 100% biome completion, because some treasures are located underwater and are seemingly inaccessible.
Unlocking the option to jump into the water
Starting from the first biome, you can find smaller and larger bodies of water. Moreover, in some of these reservoirs, treasures can be spotted at the bottom, which are also marked with icons on the mini-map. Don't jump in the water, because initially it will result in Selene's death. Fortunately, the heroine won't "fully" die. Instead, she will respawn next to the body of water and lose some of her energy.
The problem can be solved by obtaining a gadget that allows you to stay underwater. Unfortunately, this is only an option once you reach the 5th biome - the Fractured Wastes. While exploring this region for the first time, you must find the three Hadal keys necessary to open the Hadal Vault.
Inside the vault, you will find a sarcophagus and Selene will obtain a new unique gadget from the skeleton - the Hadal Ballasts These are leg-mounted devices that let you explore underwater areas.
An important tidbit is that the Hadal Ballasts are a part of your permanent inventory. You won't lose access to them in subsequent cycles. Unlocking new equipment provides you with a good opportunity to explore previously inaccessible places.
Being underwater - general information
The game will immediately force you to test out the Hadal Ballasts, as this gadget is required to reach the 6th biome known as the Abyssal Scar. You need to return to the main room in the 5th biome and jump into the circular tank.
In a similar way, you can jump from there to all of the other, smaller tanks. This will allow you to obtain loot that was previously unavailable.
The entirety of the 6th biome is an underwater area and, as a result, you will have lots of time to get used to the Hadal Ballasts. Exploration takes place in a similar way, so you still have to watch out for chasms, as falling to the bottom results in Selene's death and losing some health.
Gravity works differently underwater. Selene can jump across further distances, but at the same time these jumps are slightly slower. Keep this in mind during more intense fights. Fortunately, the Icarian Grapnel still functions the same - Selene is able to very efficiently pull herself towards all grippers. Don't worry about the oxygen supply - Selene can explore the depths without a time limit.
Underwater, you can find the same types of objects as on the surface. These include loot, translocators and interactive devices. The new environment doesn't drastically differ from the previous ones in terms of exploration.