To be honest, ultimately it's not like Poison SE Water really would do much. Poison would still be terrible except for a little "extra" for those that use STAB and Water would still be on the overpowered side, just making like two or three pokémon slightly more relevant.
In coverage Poison would be still outclassed by Electric (+Flying which is more useful than Grass, and most moves cause paralysis to Poison's... poisoning) and Grass (Ground and Rock SE are very good things).
So as much as I want Water nerfed and Poison buffed, this... doesn't do that. Water has better things it could lose and Poison better things to gain.
Not like I'd oppose to it though.
On an unrelated note, I hear Dragons. The only non-legendary, non-pseudo Dragon that is "really good" is Kingdra. Which happens to be part Water, so comparing them is a moot point.