Small details--capiz shells, a single tentacle, the fan of a fin--are a subtle way to try this trend.
Encyclopedic drawings and vintage artwork are ultra-sophisticated options, especially when grouped.
Consider a sea specimen in its habitat as a way to develop a color scheme: humpback whale gray, salt water cerulean blue, wave-break sea foam green.
Think beyond fish: Ocotopi, urchins, jellyfish, whales, crustaceans, and eels are more exotic and better representations of this trend.
Neutral colors and weathered materials, such and burlap or distressed woods, can keep this motif from feeling themey.