Tentacruel does have a good type, many common types cant do much against it, and your set looks great so far.
This is a pretty tough choice. Tentacruel is bettet at Sp. Attack than attack, so a Special Move would be good for him, whitch is why Surf is there.
I don't see much point in Rapid Spin, it is only as weak as 20 power....
You could live without Ice Beam as well actually, Surf can knock out Ground already. But of course you could use its freezing to help Hex or Venomshock
And now. Hex. Hex, Hex, Hex, Hex. HEX!!!
It isnt a bad move, but remember, the opponent isnt always on a status problem (nor is it STAB, but you don't need 2 STAB moves), but if you include a status inflicting move, it could use that advantage.... Or mabye not - Ice Beam = Freeze anyway
Venomshock is better than Hex in some ways especially if you use Toxic in your set, or just use the occasional freezing of Ice Beam. In this way, Hex is basically Tackle against Venomshock.
Also, you may not need Agua Ring if you use black sludge ot Leftovers/ Shell Bell, and with Ice Beam I would be saying goodbye to Toxic Spikes, as it provides the opponent a chance to use Thunderbolt on you.
You are right, all these moves work their own way to make Tentacruel strong, but I think the Final Set should be:
Surf - STAB, needed HM.
Ice Beam - Freeze strategey for Venomshock and fair Power.
Hex - Ice Beams freezing strategy makes this a Ghost-Type Earthquake!!!
This leaves one last space - room for Doble Team. Not Swords Dance coz' no physical moves.
Sorry if this is no-good, Ive never really made a good set.... Anyway I still hope I helped your Tentacruel...... :)?