SUMMARY/GAMEPLAY
In the future, the Earth experiences massive oceanic expansion due to climate change, with the sea swallowing up entire cities and landscapes. This provides new opportunities for deep sea research with the vast, unexplored ocean depths filled with unknown creatures, wrecked cities/towns/houses/[INSERT LOCATION HERE], and resources waiting to be uncovered. However, the deeper you dive, the more dangerous the environment around you becomes. You’re not alone down there - ancient, hostile entities lurk, and other treasure hunters are searching down there as well, some potentially hostile. The game world is divided into ocean biomes (coral reefs, deep sea trenches, underwater caverns, sunken cities, etc.). Each biome is vast and has different features, landscapes, and potentially easter eggs within them. Players need to manage oxygen levels and a food bar. (How to make a hunger/energy/water bar 🟨). Players can upgrade equipment to extend time down exploring and give themselves more oxygen or buy food for the journey. The ocean is full of strange and terrifying creatures. Some creatures are passive, while others are aggressive. Some can help the player. Players can also gather resources from shipwrecks, ruins, and ocean floors to craft new equipment, gadgets, and upgrades. The main goal is to discover treasures within ancient underwater cities and sunken vessels. Some ruins can have puzzles or challenges, and there are clues scattered around to help players piece together the history of the lost civilizations that once thrived beneath the waves. Also, add easter eggs 
. Throughout the game, players will encounter rival treasure hunters. Some treasure hunters are friendly, offering trades and quests, while others are hostile and will attempt to attack you or steal your stuff.
LORE
Due to global warming and rising sea levels, much of humanity’s history has been buried underwater. There are whispers of a secret civilization - the Oceana - a technology-advanced society that once thrived beneath the sea before mysteriously vanishing. The chracter, a former marine biologist who became a deep-sea explorer, wants to unravel the mysteries of her family’s disappearance in the ocean years ago. As the character explores, they discover strange artifacts that connect her to the lost Oceana civilization and her family’s involvement with it.
BOSSES/BOSS FIGHTS
Combat is more strategic than frantic, as players need to consider the environment, for they have to navigate through tight spaces and exploit weaknesses of enemies (Also make the weaknesses kinda obvious or no one will ever beat a boss). When going deeper, players will encounter ancient sea creatures. These bosses may require special strategies to defeat, like baiting them into caves or outsmarting them using your and their surroundings. Other treasure hunters, both sentries and possibly other players if this is multiplayer, will attempt to eliminate you and take your stuff.
MULTIPLAYER (Optional):
Cooperative Mode: Players can team up with a friend to explore the ocean together. The cooperative mode would allow players to work together on dives, share resources, and tackle challenges and boss fights together (for example, one player distracts a large sea creature while the other attacks it from behind).
Competitive Mode: Players can also fight each other, racing against each other to get treasure. This could include deep-sea races or treasure hunts, with players trying to outsmart or ambush one another in the depths.
Exploration Mode: Players can explore the world and get all the loot and stuff but no mobs spawn.
ENDGAME
As players get farther along with the game, they unlock deeper and more dangerous parts of the ocean. The final level takes them to the Abrasive Abyss, a zone where light doesn’t reach, and where the true secrets of the world’s submerged history lie. There can either be a final boss there or a game-ending treasure, you decide.
Hopefully this helps you! I spent an hour on it tonight because I have no life 
. Thanks for your consideration!