Mekkah and others, please read below for my full opinion on Tentacruel, and please read it in its entirety before judgement. Thank you.
You could start with Tentacruel or switch it in after one of your Pokemon has been knocked out. The weakness to the special attacks is nulled by its high Special Defense and possibly Mirror Coat.
Every Pokemon can lead or come in after a knock-out. It's still a terrible disadvantage not to be able to switch in on anything. See: Medicham. And you cannot really say the weakness is nullified either: it's not like you'll switch in on an attack you're going to Mirror Coat. For your information just because something isn't OHKOed that doesn't mean it's automatically not a weakness. It still takes a buttload of damage from Psychic and Thunderbolt.
As for Quagsire, Tentacruel really can't counter it unless there is an extraordinary circumstance, but that's why you have 5 other Pokemon. You also have to consider Tentacruel's resistances and advantages: it resists Surf and Ice Beam really well, it can outspeed Salamence and Flygon and OHKO them with Ice Beam, and it Hydro Pumps out stuff like Skarmory, Claydol, and company. If Zapdos chooses to come in and ruin the fun, you can Mirror Coat the TBolt back. Tentacruel will have guarenteed survival of a max SpA Zapdos Thunderbolt with just 152 HP EVs and a neutral nature. Whether these strengths will or will not override the weaknesses you've mentioned, I will not attempt to discuss, but instead finish my point.
Countering != beating it one on one. Everything except Skarmory on your list can hit Tentacruel for more than 60% damage. Mirror Coat is nice I suppose though I feel that Tentacruel is better off Swords Dancing for its sweeping potential. I perfectly know how Tentacruel is able to switch into Waters...but that's the only thing it can switch in on safely, and that's only Milotic and Suicune because Swampert has Earthquake.
Any non-Electric that is weak to Earthquake and has a low defense stat that becomes usable, or even good, in OU play certainly doesn't deserve to be in UU.
I disagree again because Tentacruel has an assload of common weaknesses and is countered by Swampert and Metagross quite easily (Metagross is not taking Hydro Pumps here...countering involves prediction).
Tentacruel takes it even further, with the potential to counter Salamence, Zapdos, Raikou, Tyraniboah, Milotic, Skarmory, Starmie, and Suicune all in one package. Such a Pokemon would almost certainly merit OU status, but unfortunately not many people realize the true power of Tentacruel, and thus,
it must be placed in BL.
Again this point is moot because most people that worked on the tiers know perfectly what Tentacruel is capable of. And once again I note I used Tentacruel a lot.
Whatever anyone says, Tentacruel is an anti-metagame Pokemon, and has the power to rock Netbattle almost as much as Tyraniboah did.
Boah could switch in more easily. =/
So, do you now think that Tentacruel still must stagnate in UU? Must Tentacruel forever give the impression of a Pokemon with a thousand weaknesses? Tyranitar is OHKOed by Brick Break and Focus Punch, and Salamence by Ice Beam, and Aerodactyl by Surf. Those four moves also see plenty of usage in the Advance metagame, but did that hinder those three Pokemon? No! People just learned how to play them properly.
These moves are all one type. Tentacruel hates 3 types of common moves, in addition to any physical attack that's at least neutral.
What's stopping people from learning to play Tentacruel properly? Consider this Pokemon is its true self, away from the dissenting opinions of anyone but yourself, and think: Does such a magnificent Pokemon, which turns its Electric weakness into an advantage, deserve such a negative public opinion. Please give Tentacruel a try, and if it doesn't work for you, then dump it, but don't trash such a treasure. Don't mistake me for a Tentacruel sycophant; I am anything but. I hated Tentacruel and still harbor a grudge against it today, but I must not let my unwilling mind cloud my speech. Tentacruel is not a god of Pokemon; in fact, it is the plainest one, and it is a greater triumph that such a mundane jellyfish and trump the greatest dragon in Pokemon. Of course, I'm not forcing you to agree with me, only expressing my personal beliefs. I hope I've made my point clear.
Your point is clear and yet it fails horribly because you automatically assume nobody else has tried Tentacruel or knows how to use it. I do and it's nothing more than a good UU Pokemon. I wouldn't die or anything if it were moved up to BL but you are grossly overrating something with such common weaknesses.