Blue Planet is a Role-Playing Game by Biohazard Games. It is a hard science fiction/cyberpunk game set on a planet known as Poseidon that is almost completely covered by water (thus, making it appear blue, and accounting for the title of the game). The game is noteworthy for its detailed and realistic future setting, with an emphasis on the environment and on biotechnology. It's also well-known for the dolphins.
The basic storyline is that in the future, humans discovered a wormhole on the edge of the solar system. This wormhole served as a seemingly naturally-occuring form of faster-than-light travel which led to another star system, of which Poseidon was one of the planets. People from Earth sent out an expedition to study and colonize the newly-found planet. Everything was all nice and pleasant and science-fictional. Until the Blight occurred. This was a global epidemic affecting food crops all over the Earth, and which nearly wiped out humanity. As a result, further shipments to Poseidon ceased, and the colonists were left to fend for themselves. Fast forward several decades. Earth is now a tattered husk of a planet, but with the Blight cured, humanity can start rebuilding itself and its planet. Once things have reached the point that space travel is reasonable again, they send out a ship to see what became of the people on Poseidon. They find that the original colonists have gone native, living a basically low-tech life, generally in harmony with the land--or rather, with the sea. Recontact is established, but things stay relatively low-key, until Xenosilicate is discovered. This mysterious ore, called Long John is found to have properties that make it crucial in certain types of biological nanotechnology--including a treatment for aging. Needless to say, the discovery of the key to immortality triggered a gold rush of immense proportions, and now Poseidon resembles a wetter, more high-tech version of the old west, complete with prospectors, outlaws, Marshals, and the descendants of the first colonists filling in for the Indians. This is where the player characters come in.
One of the best things about Blue Planet is that you can play pretty much any type of character that's appropriate to the setting. From a native sellout street urchin, up to a biologically-modified supersoldier. You can even play a dolphin or killer whale, or a biotech human-animal hybrid. The character creation system is detailed, but generally leads to fairly realistic characters.
In both editions of Blue Planet, the rules are fairly simple, but versatile, but with a wealth of options (especially in combat) that sometimes seems a bit overwhelming. However, the emphasis is seldom on the rules themselves, but on the characters and setting.
As I mentioned before, there are two editions of Blue Planet. The first edition is by far the prettier of the two, although the interiors are nearly devoid of art. All the crucial information is contained in one book, with a supplement (Archipelago) containing information on more islands on Poseidon. The rules are percentile-based, and vaguely reminiscent of Call of Cthulhu or a simplified Rolemaster. Combat is rather complex, with time measured in half-second rounds, and detailed wound charts based on hit location, which are somewhat similar to Rolemaster's infamous critical hit charts.
Second edition Blue Planet has less pretty, more cartoon-like covers, and more (but lower quality) interior art. There are two core books, the Player's Guide and Moderator's Guide. As the names suggest, the PG is designed for players, and the MG for "Moderators," or Game Masters. The PG contains all the rules, while the MG has mostly setting information, as well as stats for creatures of Poseidon, and tons of adventure seeds. While the rules have changed, the setting is identical, and most of the information in the MG (and a lot in the PG) is reprinted from the First Edition Blue Planet book, or from Archipelago. There is a supplement for Second Edition, called "Fluid Mechanics" which is about technology in the Blue Planet setting, and has additional equipment and such. The Second Edition rules have been greatly streamlined, and are now based on a single d10 (or two or three, using a clever game mechanic called "aptitude" in which characters may specify certain groups of skills that they essentially get one or two extra chances to succeed at). Combat is almost totally changed, replacing the hit location charts with a mechanic that deliberately abstracts hit location. It's simpler to understand, and faster in play, yet doesn't really distract from the realism (or the lethality--both versions of combat are deadly!).
So, in closing, I heartily recommend either edition of Blue Planet. The first edition should be pretty rare by now, but it's still very good (if you don't mind the Rolemaster-like combat), but the second edition shouldn't be too hard to find. (Wizards of the Coast stores carry it, I know.) If you like hard science fiction or cyberpunk, detailed settings, realistic characters, and dolphins, then Blue Planet may just be the RPG for you.