I'm going to be comparing this to some of the S tiers in my assessment. I generally compare S tiers more than most as they really need to be a clear stage above most other things and a power level
Positives:
-By far the best starter for Brock, soloing at an early level (despite all starters beating him).
-Pretty good for Misty, though 6HKOing Starmie with Mega Punch is really a drag.
-Despite the negative Lt. Surge matchup, you can still take out everything but Raichu as Voltorb and Pikachu can't threaten you.
-Fairly self-sufficient movewise, only really needing BubbleBeam and Mega Punch, which are fairly uncontested.
-Fantastic for all Giiovanni battles, Blaine and Bruno. Pretty good for Lance too, though a sweep may be shaky due to Hyper Beam spam.
Negatives;
-Has some power issues pre-BubbleBeam, as Mega Punch is generally better than Water Gun to achieve 2HKOs. Even with BubbleBeam, unlike Kadabra's Confusion and Nidoking's Thrash, Wartortle is likely to be barely 2HKOing most things compared to the other two who generally knock their targets to red if not outright OHKO them.
-Water is a pretty popular type on the mook base, and Squirtle generally doesn't like them and the occasional Grass types. Right after you get BubbleBeam on S.S. Anne, you are forced right back into missing Mega Punch again for a bit, and you also have some Swimmers/Fisherman near Cinnabar that will force you to spam non-STAB Strength. This sort of mediocrity with the mooks isn't a one-time thing - it also doesn't like the Gastly line in Pokemon Tower / Agatha with their high Special and the Abra line (most notably Alakazam) can be annoying to kill reliably because of Reflect and Recover.
Midgame is fairly better mook wise as you can spam Surf after you get it, but then Blastoise has some issues with the Psychic types in the Fuschia and Saffron Gyms. Granted, Psychic is broken and causes issues for almost everything. Against almost everything else? Yeah it generally 2HKOs with Surf, which is fine, but it almost never OHKOs - for example, even something like a Viridian Gym mook with Machoke lived Surf in red IIRC.
-I could overlook the meh mook base if Blastoise was excellent in bosses. While Blastoise turns in a pretty good showing, I don't think it's S. As stated, it takes forever to kill Misty's Starmie (granted, even a slow but good matchup is kinda above average there). It's okay for Surge, dealing with the small stuff, but not the ace mon (turning in a similar performance for Sabrina, mostly due to her weak moves).
Wartortle outright folds to Erika more or less, even with Ice Beam (she deals too much damage). Blastoise sweeps Koga but Smokescreen can cause issues and it doesn't even OHKO his Koffing with Surf, again accentuating the lack of power. It sweeps Blaine and all of Giovanni's battles with virtually no issues...
...but then it runs into some issues at the League with both Agatha (I'm gonna assume you don't give this thing Dig, as that's the most contested physical TM in the game and all uses of it for Wartortle are too niche to really justify the cost in my opinion) and especially Lorelei. It sweeps Bruno, but so does literally anything else just about, and its performance against Lance isn't as watertight as the Ice Beam access may suggest due to level disparity and Hyper Beam spam wearing it down.
Champion Blue also leans slightly average for Blastoise, as Venusaur, Alakazam and (usually) Gyarados can easily overpower it, though it beats Pidgeot, Arcanine and Venusaur fine.
Overall assessment:
Squirtle is a great starter,
but I don't feel like it's on the level of Abra and Nidoran Male (I still need to test Nidoran Female) and is a good fit for A tier. It's a clear step behind both in the mook base and Squirtle generally doesn't sweep as many bosses as quickly as the other two do (despite Nidoking sharing meh matchups for Misty and Erika, it makes up for this by sweeping the second half of Gyms pretty reliably). The Squirtle line 2HKOing things in general is fine, but a S tier generally needs to be scoring a lot more OHKOs, which the line fails to do most of the time unless it's hitting super effectively with BubbleBeam/Surf. While Squirtle's TM dependency is better than Nidoking's but behind Abra, which is pretty good, it still fails to make up for the lack of power Squirtle generally has throughout its life.