First you gotta find it.

It's one thing to identify what you see underwater, but first you have to find it. Locating marine life is all about spotting patterns. Certain things tend to be in certain places. Dive slowly. The slower you dive, the easier it will be to find things.

From easiest to find to most difficult:   

CONCH: Look under big shells in the sand patches  

EELS: Look in turtle grass for sharptail eels. Check lettuce coral for Goldentail eels. Green morays are often found tucked into the reef, along the walls, with most of their bodies hidden and only their heads exposed. You can also find them underneath ledges on the reef, the same place you'd like for nurse sharks or maybe even a toadfish if you're lucky.

NUDIBRANCHS and LETTUCE SEA SLUGS: Check coral rubble, especially in canyons or swim-throughs. Or dive at night. Sea slugs and nudibranchs are much easier to find at night. You can also find lettuce sea slugs in lettuce coral, where you might find the Goldentail eel, reef urchin, or small crabs (e.g.
Gaudy Clown crabs). You can also find nudibranchs in the turtle grass, especially when it's flowering. 

SEAHORSES: Check soft coral and look carefully. Oftentimes they take on the colors of their surroundings, making them difficult to find.

BARRACUDAS: Look behind you!