The Food and Agriculture Organization of the United Nations has several nice, downloadable, species identification books available for free.
You could start at: FAO Document Repository and search on the word identification, (then scroll through lots of other stuff)
or go directly to some of these examples:
marine mammals
Marine mammals of the World
fish, etc. of the Western Central Atlantic (includes Caribbean) in 3 volumes
The living marine resources of the Western Central Atlantic. Volume 1: Introduction, molluscs, crustaceans, hagfishes, sharks, batoid fishes, and chimaeras
The living marine resources of the Western Central Atlantic. Volume 2: Bony fishes part 1 (Acipenseridae to Grammatidae).
The living marine resources of the Western Central Atlantic. Volume 3: Bony fishes part 2 (Opistognathidae to Molidae), sea turtles and marine mammals
sharks
FAO species catalogue Vol.4. Sharks of the world. An annotated and illustrated catalogue of shark species known to date. Part 2. Carcharhiniformes
sea turtles
FAO species catalogue. Vol.11. Sea turtles of the world. An annotated and illustrated catalogue of sea turtle species known to date
etc.
You could start at: FAO Document Repository and search on the word identification, (then scroll through lots of other stuff)
or go directly to some of these examples:
marine mammals
Marine mammals of the World
fish, etc. of the Western Central Atlantic (includes Caribbean) in 3 volumes
The living marine resources of the Western Central Atlantic. Volume 1: Introduction, molluscs, crustaceans, hagfishes, sharks, batoid fishes, and chimaeras
The living marine resources of the Western Central Atlantic. Volume 2: Bony fishes part 1 (Acipenseridae to Grammatidae).
The living marine resources of the Western Central Atlantic. Volume 3: Bony fishes part 2 (Opistognathidae to Molidae), sea turtles and marine mammals
sharks
FAO species catalogue Vol.4. Sharks of the world. An annotated and illustrated catalogue of shark species known to date. Part 2. Carcharhiniformes
sea turtles
FAO species catalogue. Vol.11. Sea turtles of the world. An annotated and illustrated catalogue of sea turtle species known to date
etc.
