I run Max Def Tentacool and I've been very happy with the results. I run a Rapid Spin/ T Spiker set with Boil Over and Acid Spray. I know Tenta's SpA is god awful but I use it mostly for the burn hax if I don't have T Spikes down, and after a few Acid Sprays it does good damage.
His typing is mostly what he has going for him. A Defensive Poison/Water type is an absolute gift in NU, and can put a full stop to the very annoying Roselia, spinning away/ absorbing Spikes/T-Spikes, can't be poisoned, and Giga Drain and Leech Seed damage back due to Liquid Ooze.
For an idea of bulk, it takes roughly 80% from a scarf Braviary's Brave Bird, which is quite good.
The main problem I've encountered with it is that it tends to be set up bait for things with sub/ special attackers so Haze deserves a big mention.
I'm actually really dubious of this, can you provide some calcs? I'd like to know what exactly it can switch in on and threaten enough to spin. In addition, it needs to carry eviolite, which means you're relying on using it as a "revenge spinner" of sorts, or carrying a pokemon with wish (not a bad thing, just not ideal).
Here are some calcs I've run, noting that a lot of them rely on spikes/sr being up becuase Tentacool is too frail to be taking a lot of needless hits and you should only be using it to spin:
252 SpAtk Life Orb Magmortar Fire Blast vs 252 HP/4 SpDef Eviolite Tentacool: 29.23% - 34.51%
252 SpAtk Life Orb Magmortar Thunderbolt vs 252 HP/4 SpDef Eviolite Tentacool: 61.97% - 73.24%
That's a KO with rocks/spikes.
0 SpAtk Tentacool Scald vs 0 HP/0 SpDef Magmortar: 36.08% - 43.3%
That's the reverse.
252 Atk Braviary Brave Bird vs 252 HP/252 Def Eviolite Tentacool: 62.32% - 73.24%
Adamant Braviary vs Tentacool: it survives but it can't switch in on it.
252 Atk Choice Band Sawk Close Combat vs 252 HP/252 Def Eviolite Tentacool: 42.96% - 50.7%
2hko'd with rocks/spikes but i guess gets the spin off?
252 +1 Atk Klinklang Return vs 252 HP/252 Def Eviolite Tentacool: 45.77% - 54.23%
again, a 2hko with rocks/spikes.
I'm not really sure what the point of it is when there are other better spinners in the tier.
edit: seriously where are all the eelektross, for a #3 used pokemon I have seen exactly one.