Underwater Base: A sprawling, multi-level base built into the side of a cliff or mountain, incorporating underwater tunnels and viewing areas. Use sea lanterns for lighting and prismarine, dark prismarine, and sea pickles for a vibrant, aquatic aesthetic. Include a kelp farm for renewable resources and a breeding area for aquatic creatures. Consider adding a hidden entrance disguised as a natural rock formation.

Underwater City: A bustling metropolis built within a large, enclosed underwater structure. Design individual buildings with unique architectural styles, using glass panes for windows to showcase the interior and the surrounding ocean. Incorporate bridges and walkways connecting different sections of the city. Include a central plaza with a large, decorative fountain and public spaces for your aquatic citizens. Consider adding a marketplace and various shops.

Underwater Temple Expansion: Take an existing underwater temple and expand upon its design. Add additional chambers, secret passages, and treasure rooms. Reinforce the structure to prevent collapse and incorporate defensive measures against hostile mobs. Use a mix of natural and man-made materials to create a blend of ancient and modern architecture.

Underwater Farm: A self-sustaining farm located within a large, enclosed underwater structure. Use bubble columns to create air pockets for crops and livestock. Include different farming sections for various plants and animals. Consider adding a system for collecting and filtering water. Use glass panes to allow sunlight to reach the crops.

Underwater Ruins: Create a mysterious and ancient underwater ruin, hinting at a lost civilization. Use weathered stone blocks, mossy cobblestone, and cracked stone bricks to create a sense of age and decay. Incorporate overgrown coral and other aquatic plants to enhance the natural feel. Include hidden chambers and secret passages to add to the mystery. Consider adding chests containing valuable items or clues to a larger story.

Submarine Base: Build a mobile underwater base in the form of a submarine. Design the interior with various rooms, including a control room, living quarters, and storage areas. Use iron blocks, dark prismarine, and glass panes for a sleek and modern look. Include a propulsion system and maneuvering controls. Consider adding defensive weapons and countermeasures.

Underwater Aquarium: Create a large, enclosed aquarium showcasing a variety of aquatic creatures. Use glass panes to create a panoramic view of the inhabitants. Include different sections for various species, with appropriate lighting and environmental features. Consider adding informational signs and interactive elements. Use a variety of coral and other aquatic plants to create a vibrant and engaging ecosystem.

Underwater Volcano Base: Build a base within or around an underwater volcano. Use lava, obsidian, and netherrack to create a dramatic and fiery aesthetic. Incorporate lava pools and waterfalls for visual interest. Consider adding a system for harnessing the geothermal energy of the volcano. Use reinforced structures to withstand the heat and pressure.

Remember to consider lighting, accessibility, and defense mechanisms when designing your underwater build. Use sea lanterns and glowstone for illumination, and incorporate traps and defensive structures to protect against hostile mobs.