Tentacruel is viable and a bit of a side-project I'm working on. Base 100 speed means it can outrun a lot of things, nice if you want a fast Regenvester over a slow one, and it has great special bulk combined with some key resistances. Observe...
252+ SpA Choice Specs Pixilate Xerneas Boomburst vs. 252 HP / 252+ SpD Assault Vest Kyogre: 186-220 (46 - 54.4%) -- 55.9% chance to 2HKO
252+ SpA Choice Specs Pixilate Xerneas Boomburst vs. 252 HP / 252+ SpD Assault Vest Tentacruel: 105-123 (28.8 - 33.7%) -- 0.9% chance to 3HKO
And for fairness, against a common resistance...
252+ SpA Choice Specs Refrigerate Kyurem-W Boomburst vs. 252 HP / 252+ SpD Assault Vest Kyogre: 114-134 (28.2 - 33.1%) -- guaranteed 4HKO
252+ SpA Choice Specs Refrigerate Kyurem-W Boomburst vs. 252 HP / 252+ SpD Assault Vest Tentacruel: 127-150 (34.8 - 41.2%) -- guaranteed 3HKO
Against shared resistances, the bulk is a little worse, but Tentacruel is still safely out of 2HKO range from one of the nastiest special attacks in the tier. And the key thing is that Xern calc, which is something that'd give Kyogre trouble. Really, the draw here is Tentacruel's resistances from its Poison-typing, which lets it resist Fairy, Poison (some Gengar sets), and Fighting while having a Grass neutrality (handy against Leaf Storm Contrary, which is sometimes a thing specifically to piss off opposing Kyogre). Psychic weakness kind of stinks, but Kyogre can't take Psystrikes well at all either, so the only real loss is to Psycho Boost. A weakness to Earth Power also is kind of meh, but most Ground is physical anyway, which again, Kyogre isn't too fond of either.
The standard Scald/Volt Switch/Coverage or Nuzzle or Dtail or Spin/Repeat slot 3 set AV Kyo runs works pretty well, though I've been trying some other things with it too. Sludge Wave threatens opposing Xern and other, less common fairies rather well. Acid Spray is fun and great for forcing a switch. On a non-AV set, perfectly accurate Toxic is fun and often unexpected. And apparently it can hit through stuff like Shadow Force, though the only team I'm aware of that'd be useful against is Flint's AssistArrowave team.
If you have a Fairy weakness on a team, I recommend giving it a shot. Its surprising at how well it can take hits. Just don't try to run an offensive set on it. Tentacruel's power, while decent in standard, is a bit lacking for offensive roles, so keep it relegated to defense and support.