One of your best options would be your starter. Mudkip evolves into first Marshtomp, and then Swampert. It has quite decent defensive bulk, as well as having a very unique defensive typing. Additionally, it has received a Mega Evolution, increasing it's offensive capabilities.
Since you are playing AS, you will have Lotad available pretty early on in the game. It evolves into Lombre, then Ludicolo, who has very nice options for for Rain teams, and some interesting utility moves. These make it a great sustaining annoyer.
Marill is another option available relatively early, and evolves into Azumarill. In addition to being a great bulky water type, it has also had the Fairy type added to it, letting it hit Dragons where it hurts. Make sure you get one with the Huge Power ability, though.
Finally, as you are playing AS, you will have Kyogre made available to you as you play through the story. Being a Legendary Pokemon, it is a veritable force to be reckoned with. Additionally, it has received a variation on Mega Evolution, known as a Primal Reversion.
INB4 "7.8/10 too much water", the Hoenn Region has historically gotten a lot of flak for it's many waterways. There are a number of Water Pokemon that become available as you near the end of the story and approach the Pokemon League.
