Personally, I would go for Gastrodon. It's ability Storm Drain is, in my opinion, way more useful than Swift Swim in a playthrough team, as rain isn't the most common thing without Drizzle or Rain Dance. You can get Shellos just before the first gym if my memory serves me correctly, which can definitely help later in the game if you switch it in after your to-be Infernape to try to bait a water move (I hope that sentence makes sense). When it evolves, its added ground typing removes all but one of its prior weaknesses, with added coverage that can be a huge aid with gym leaders such as Byron or even the Champion. The only gripe I have is it's speed, but because it can take any hit that isn't a Grass move, it's hardly an issue, especially with your starter choice. I used a Gastrodon on my BD team and it was the star of the show other than my own Infernape.
If you need a final moveset, here's what I'd consider;
Scald/Surf
Earth Power
Ice Beam
Recover
I also ran Leftovers once I had access to those.
Good luck! You'll need it towards the end. (If you know, you know.)
Edit; turns out you can get a pink one on route 205, just after the Valley Windworks, if not than just before. I got a blue one, but the choice is yours.
