Water is the most calm and free-flowing elements. These characteristics dictate its nature, making it useful in combat due to the fluidity and intense lack of control that is required to utilize it. Lightning and Fire take extreme control, but manipulation of water takes calmer movements and so controlling it comes more naturally.
While its the calmest element, it can also become one of the most destructive. Putting a large amount of force behind of wave gives it power that can't be paralleled by all the other elements. In terms of scale, Water has the highest potential to be the "best" element, a clear testament of this is Kisame of the Legendary Akatsuki. RIP Kisame . My favourite character died too soon.
Water is weak to Earth due to the solid element dissipating the force of the waves, essentially nullifying any effect they could have on the opponent. Water is strong to Fire because it extinguishes the flames. Common sense, i know, but it has to be said.
Water and Lightning interact in a very odd way, most of the other elemental relationships are straightforward but these two insist on complicating things xd . Water conducts electricity (Lightning) and so in a way can have a strength over Lightning as it can redirect it back onto the user. Lightning also gives an element of lethality to Water Techniques, something most of them lack in.
Water and Wind simply react in a neutral manner to eachother. Wind seeks to overpower the water, Water seeks to overpower the wind. When they are used on equal ranks, they dissipate eachother's force entirely.
Kekkei Genkais that rely on water include: Storm Release (Lightning + Water), Wood Release (Earth + Water), Boil Release (Fire + Water) and Ice Release (Wind + Water).
Come to think of it, this element is pretty damn useful. It forms a KG relationship with all the other elements and can even make Mud Release when used with water.
I would mainly use water to create mud offensives, redirect Lightning (to counteract my Earth weakness), take out Fire Release techniques (S-rank), give my water mask some jutsu to use and to put pressure on highly intensive users of Lightning and Fire. .