Is tentacruel actually worth using in OU singles?

It is passive, that's sort of the point of it. It's there to spin, absorb toxic spikes and sit on special attackers with that big special defence, not to threaten anything. On rain though you've got a free win: run Rain Dish. With Black Sludge on top it heals a decent chunk every turn in rain. Moves I'd look at are Rapid Spin, Knock Off, Haze or Toxic Spikes, and a water move (Surf, or Flip Turn if you want the pivot). For EVs, max HP and put most of the rest into whichever side your team is weaker on, usually defence because the special side is already good.