General:
* Aquatic: relating to water, especially water that is not salt water.
* Oceanic: relating to the ocean.
* Nautical: relating to ships, sailing, or navigation.
* Maritime: relating to the sea, especially in connection with shipping or trade.
Specific:
* Marine: (the most direct equivalent, but can also refer to sea life)
* Submarine: relating to the underwater world.
* Coastal: relating to the coast.
* Lagoon: relating to a shallow body of water separated from the sea by a narrow strip of land.
Figurative:
* Salty: used metaphorically to describe something related to the sea or having a rough or harsh quality.
* Deep-sea: referring to the depths of the ocean.
The best adjective for you depends on the context. For example:
* "Marine life is fascinating" - referring to sea creatures
* "The city has a strong maritime history" - referring to seafaring and trade
* "The coastal town is a popular vacation destination" - referring to the location near the sea
Consider the specific meaning you want to express to choose the most appropriate adjective.