Wouldn't Hidden Power [Grass] or even [Electric] work better on Tentacruel?
Plus Electric hits Gyarados especially hard, you know, the ones who try and Taunt or Dragon Dance instead of use Earthquake?
Plus if it makes a difference, Charizard uses Hidden Power Grass, so you have less need for another Grass Attack.
Sure Giga Drain heals, but not by much with its low base power, and your minimal Special Attack EV investment, even when super effective (except vs Swampert).
Overall go for HP Electric. Also, whenever I use Tentacruel/Vaporeon I always use Hydro Pump for more power, and it pays off. It does wonderous damage to Lucario, Gengar, etc.
I often use Tentacruel to take on Lucario (as it resists Close Combat), and since it usually goes 2nd to Lucario, the Close Combat Special Defense drop really helps to let Hydro Pump go for the surprise KO.
Hydro Pump also does alot to even Spiritomb and Dusknoir. And consider Toxic Spikes over Sludge Bomb, handles Blissey, and other walls nicely.