Plesioth Drencher is the current highest damage water element light bow gun in the game. A close second is the Jade Tempest, which has sleep, instead of paralysis.
While solo, you might want to sleep-bomb instead of paralysis, however, in a group, hands down, the paralysis outweighs the sleep... since not every hunter is wise enough to step away when they see the monster go to sleep.
Plesioth Drencher is a favorite of mine against Silver Rathalos. Specially when you have the Bonus Shot, and Water Damage +2 active. There is just a great feeling of shooting the los out of the sky, and have it go into paralysis while you shoot it in the face with water :-D