So, what is Water-type's role, really?
The Legendaries and Their Role:
Said to be incarnation of the north wind and representing the compassion of pure water, Suicune has the power to purify murky waters. Suicune's design seems to be based off Qulin, and its association with north wind seems to be based off Fujin, Shinto god of storm, which is also represented by Tornadus.
Said to be the creator of the sea, and bringer of tidal waves and rain, Kyogre is the personification of sea. According to Sapphire item in FRLG, Kyogre also represents honesty. Kyogre is based off Leviathan, specifically its Arabic version, which depicts is a large fish or whale.
Said to be Prince of the Sea, Manaphy can connect with anyone by switching its consciousness or others consciousness. Its red part is based off glowing red center of sea angels, but its' gem might be a coral pearl.
Based off Kanaloa, Hawaiian goddess of ocean and underworld, Tapu Fini represents death. It is based off a mermaid and its mask is based off a swordfish.
Though Bulbapedia claims otherwise, Palkia is more likely based off offspring of Izanami, specifically Susanoo-o, than Izanami or Izanagi. Its pearl is likely a reference to "pearly gates", biblical definition of Heaven. Its association with Water likely represents the expansion of Space. Palkia's phallic shape might also represent "World Parent Creation Myth" archetype, that often depicts an union between by Sky and Earth.
Based off d'Artagnan, the young recruit of the Three Musketeers, and a generic kelpie and a kirin(Chinese version of Qulin), Keldeo appears in scenic waterfronts. It is likely meant to represent youth.
Same as in Fire-type analysis page. Actually, don't, since Volcanion also represents geyser/volcanic fog and steam in addition to what's written there.
The Trainers and Their Roles:
Nicknamed "The Tomboyish Mermaid" she is a swimmer. Her signature Pokémon is Starmie. She has a hero worship on Lorelei, further tying Ice and Water-types with each other, and Ice's presumed role as the upgrade of Water-type.
Nicknamed "The Gym Leader with the beauty of pure water!" he is the teacher of Wallace. His signature Pokémon is Kingdra.
Nicknamed "Artist, and lover of water." His signature Pokémon is Milotic. His take over as the Champion likely references Water's resistance towards Steel, and weakness of Steven's team to Water-types.
Nicknamed "The Torrential Masked Master", despite being designed as a western Pro Wrestler, he is actually a performer. His Signature Pokémon is Floatzel in most matches, and Ludicolo in Platinum rematch. His association with pro-wrestling likely references Water's association with Fighting.
The confident one of Straiton Triplets, his Signature Pokémon is Simipour.
"A Bigger Splash Than the Sea!", he is based off a male swimmer and a net fisher. His signature is Male Jellicent. He has a Kyushuan accent in Japanese, likely intended to match its Southeastern Asian profile (in reality, that's because he has a tan).
Based on a chef, his Signature Pokémon is Barbaracle. He is the most serious of Water-type trainers.
Based on a fisher, her Signature Pokémon is Araquanid. Despite her shy appearance, she often tries to trick player into believing her ridiculous claims and often speaks in a overly sophisticated manner.
Despite having a range of personalities, Water-type trainers are generally fun loving and calm people. Two of them are based off swimmers, two of them are based off cooking, two of them are based off fishers, and three of them are based on performers. The versatility of Water-types are represented on the differences of Water-type trainers.
On the case of Legendaries, while the primary weight goes to sea deities, there are others associated with rivers, storms and geysers. This is likely because there is more sea based Pokémon than other water-type based species. Despite being effectively a sub-type of Water and Ground there is no Mud-based legendary or trainer.
Because Water-type has had a representation in all regions regardless of Water-type distribution, it is likely that Water-type will get a full array of legendaries and trainers in the future.