A possible match for the criteria in question would be Blue Planet. Several of the character options are for cetacean characters (not actual fish, but perhaps still relevant).

Primary themes of the game are exploration, survival, and making your place in the world. The game offers a lot of detail on inter-species relations, as well as tools for maximizing player awareness of cetacean culture. The backdrop, of course, is one of the more detailed and layered SF games out there, and its strong focus on a marine environment.

The game is available in PDF in all three of its editions. A fourth edition is pending, but will be based on the game's 2nd edition. Both 2nd and 3rd (Revised) editions are solid and easy to learn. Revised has the advantage of shrinking the game line into 3 books, without dropping any of the system, character types, or gear. The setting material has remained consistent across all editions.

Advantages:

• Marine mammals as characters are a primary part of the system and setting
• Experiencing play as marine mammal is the focus of an entire sourcebook
• SF options for interacting with humans, technology, etc are present allowing for a contrast between worlds as part of RP. These can be ignored.

Disadvantages:

• The game assumes you will play genetically up-lifted marine mammals
• The game is not set on Earth [easily adapted, but adaptation might be required for the Original Poster]
• The game is technically between Editions

A good match for the criteria in question would be Blue Planet. Several of the character options are for cetacean characters (not actual fish, but perhaps still relevant).

Primary themes of the game are exploration, survival, and making your place in the world. The game offers a lot of detail on inter-species relations, as well as tools for maximizing player awareness of cetacean culture. The backdrop, of course, is one of the more detailed and layered SF games out there, and its strong focus on a marine environment.

The game is available in PDF in all three of its editions. A fourth edition is pending, but will be based on the game's 2nd edition. Both 2nd and 3rd (Revised) editions are solid and easy to learn. Revised has the advantage of shrinking the game line into 3 books, without dropping any of the system, character types, or gear. The setting material has remained consistent across all editions.

In its unmodified form, Blue Planet is set in the future, and on another world. That said, there is nothing to prevent a game focused on life in the oceans of that world, or any other you devise. In fact, there is substantial material to support such a focus.

Advantages:

• Marine mammals as characters are a primary part of the system and setting
• Experiencing play as marine mammal is the focus of an entire sourcebook
• SF options for interacting with humans, technology, etc are present allowing for a contrast between worlds as part of RP. These can be ignored.
• Core Books (Player's Guide, Moderator's Guide) are detailed and sufficient for play, especially in the Revised Edition which incorporates earlier sourcebook material directly

Disadvantages:

• The game assumes you will play genetically up-lifted marine mammals
• The game is not set on Earth [easily adapted, but adaptation might be required for the Original Poster]
• The game is technically between Editions
• This is not a single book purchase (minimum of 2)

Links:

Wikipedia (accurate)

DriveThruRPG (1st, 2nd, Revised Editions)

Blue Planet Revised Overview

Ancient Echoes Sourcebook Overview (Cetaceans in-depth)

Natural Selection Sourcebook review (Ecology in-depth)