I just thought I'd put this out there for everyone so I can explain myself. This isn't just because of my love of most of the fire-type starters. It actually has a lot to do with the fact that fire-types can be more difficult to obtain otherwise, becoming more of a business decision. While Oshawott was indeed my favorite Unova starter, I felt that my White 2 run overall would be better served by picking Tepig. Why? Because I knew I could simply go to the Driftveil Drawbridge and catch a Ducklett, if necessary, to be my primary water pokemon. In the SoulSilver run I'm currently playing through, I once again picked the fire type over my regional favorite, Totodile. The only time, thus far, that I went ahead and took the water pokemon was in Y. And that was because I knew I could just go on ahead and pick Charmander later, combined with the uncertainty over whether or not a fishing rod will be available before the Elite 4 (yes, I started a Nuzlocke run right from the beginning when only minimal information was available).
And yes, there's the main reason I don't usually start with water type pokemon in nuzlockes. Sure, there are some regions where this makes a huge difference, like Hoenn, where this is the safest way to obtain the well-loved Water/Ground combo. Or perhaps a region like Unova, where the Old Rod and Good Rod are never handed to the player. The main reason I don't usually start with water pokemon is the Old Rod and Good Rod being so easy to get, and fishing in town doesn't count towards an encounter on one of the adjacent routes. Going deeper than that, is when some decent water types can be found simply by surfing. As much as I like the Totodile line, I would accept, in terms of gameplay, if I instead had to beat up opposing fire pokemon with a Gyarados or Poliwrath rather than Feraligatr as opposed to failing my few chances at catching a fire pokemon.
Then, there comes the issue with most grass-types. They're usually easier to seek out than fire types. Maybe not the case in Kanto-based games, where Kindle Road and Pokemon Mansion are wonderful places to catch a fire pokemon in the mid-to-late game. However, grass pokemon are almost as consistently available surrounding Cerulean City. Most players are able to catch an Oddish/Bellsprout on their way to the cape. In Hoenn, once again, we encounter an early game that makes it easy to obtain a grass-type quickly even if you didn't pick Treecko. Headbutting and coating trees in honey can be really great ways to lure in specific grass pokemon if one proves necessary, but the forests in the games that feature those methods ALSO have some nice encounter rates for grass pokemon.
It seems that often, fire pokemon are hardest of the three starter types to obtain in the wild. ESPECIALLY in Diamond and Pearl, where slim pickings means players who do not start with Chimchar are either left with just Ponyta or must attempt to compensate for a lack of fire with other types. Kanto, Hoenn and Unova provide the best chance to obtain a fire pokemon that isn't your starter. Johto has passing opportunity for players who started with Totodile or Chikorita to catch a Vulpix/Growlithe. Let's say a player who started with Chikorita goes to Route 36 only for a male Nidoran to show up first (not caught on route 35, so the most common form of dupes clause can't be invoked). Later, at Route 37 once Whitney is finally defeated, Stantler jumps out to be the first encounter (While Stantler is rare on Route 36, it also hasn't been caught as of yet, so dupes clause again cannot be used). This means the player now has to either find a fire stone for the Eevee obtained from Bill by smashing countless rocks or wait until the slim encounter rate for Ponyta on the approach to Indigo Plateau.
If you don't feel like listening to my advice, good for you, too. The one time I actually did pick the water pokemon for a nuzlocke, I wound up enjoying myself and getting attached to Bud the Greninja, to the point where that experience caused Froakie to surpass Fennekin in my rankings for Kalos starters. If I had known about the old rod and good rod being available when I started the game, I would have run with Fennekin and Bulbasaur instead. Thus, I never would have gotten the opportunity to play using Greninja.
P.S. My Greninja's nickname is based on a certain commercial I found humorous when I was a kid. If you grew up in the 90s, you should remember it.