I just finished my water mono type run,and it seems one of the Pokemon that helped me out a lot early/middle of the game isn't here,so I'm gonna do him justice!
Dewgong:(Water) Class -C/D
Abilities:Hydration/Thick Fat
Movesets: Perish song,Surf,Ice Beam,Sheer Cold/Frost Breath
Reason:While some people may laugh at the face of Dewgong,you might be surprised at what it can actually do,before we get into his advantages,lets talk about his disadvantages.First of all,you can "Have" a chance of either getting a Canea(If I am not mistaken)or a Seel egg,so thats a 50/50 chance to get him.Second,His stats arent quite that high,with a total of 475.Now lets talk about his advantages,You can get him as soon as you kill the first Pulse-Tangrowth,which is a bit early than any other ice types in the game (I think).Secondly,for a water mono run,the second gym leader and those pulse-Tangrowths can be a huge problem,but thanks to Seel having a chance to get perish song,its pretty much a free kill.This actually helped me beat the "Impossible" Bosses,you just need to sash it and wait for 3 turns than boom,the threat is gone.Lastly(this is all that i could think of,maybe i'll edit something later.),This is Pokemon Reborn,Hax are bound to happen every battle and because this is Reborn,it also rain 24/7,Dewgong gets to counter hax with its ability,hydration which removes all status that is inflicted,such as poison and burns when rain is active,this can be very helpful if your very unlucky like me ( )and its great for a rain team,Oh and I almost forgot,with your rival,Fern using grass types,your going to be needing a counter for that,and Dewgong with its ice typing comes into play.The reason why Dewgong isnt higher on his class,beside his disadvantages i have written above,Dewgong gets out classed by many Water Type that you get Late/Mid game such as Lapras,Heck,even Swanna is better than him,but still,theres no other early game poke that gets perish song, so thats got to be a plus right?