Despite its OK base 85 special attack, most people focus on Cloyster being a physical tank, and therefore on its physical attack. However, as Shell Break raises both physical and special attack, a special set is more than viable, especially to beat all those physically defensive Steel-type counters to most normal sets.
EVs (preliminary): 6 HP, 252 SpAtk, 252 Speed
Ability: Shell Armor
Item: Focus Sash/Rechargeable Battery
Signal Beam/Mud Shot/Icy Wind/Explosion
A fairly standard 'Bulky Water' moveset, other than the facts that Cloyster has physical bulk rather than special, has STAB with Ice-type moves and knows Shell Break.
A Rash nature (+special attack, -special defence) should be run to give Cloyster 295 SpAtk, boosting the power of its moves. A Naive nature (+speed, -special defence) can be run instead if you want to beat Choice-Scarfed Dugtrio, Jirachi and Flygon after one Shell Break. Just remember that even before Shell Break, your special defence is pitifully low, so keep away from special sweepers. Also, if you want to lower your defence instead (not advised), you can use a Mild or Hasty nature.
As an ability, Shell Armor is preferred, as it prevents game-breaking critical hits (especially from priority special moves like Vacuum Wave). However, if you plan on switching Cloyster into a Sandstorm, Overcoat can be used.
For the item, Focus Sash can be used to take a special hit and retaliate with Explosion. However, Rechargeable Battery grants immunity to one Electric-type attack, raising the attack stat one stage. This can be used to take down a surprised counter, and could even allow the use of a more powerful explosion with the +3 attack (after Shell Break). Of course, Cloyster is unlikely to outspeed much before using Shell Break, which would hinder its chances of pulling off a successful Explosion.
Shell Break is the most important move on this set; a turn of setup is really needed to do a decent amount of damage and outspeed the opponent.
Surf is the most common special Water-type move, and for good reason: 95 base power with 100 accuracy gives a reliable, reasonably powerful move. It is perfectly fine for taking out most Steel-types, however Hydro Pump can be run over Surf, and while it has an extra 25 base power, it only has 80 accuracy and is a bit unreliable.
Ice Beam is the same as Surf, apart from being Ice-type and having a pitiful (but awesome if it works) chance of freezing. Purely for type coverage and STAB. Blizzard can be run for more power if Cloyster is on a hail team.
The fourth move is mainly filler, but you can use Signal Beam as a decent Bug-type move, to hit Dark- and Psychic-types Super Effectively and get neutral hits on Water-types. However, it only does a bit more damage with a SE hit than Surf does after STAB. Other Pokemon can do better jobs of countering Water-types, and this combination of types is resisted by Empoleon, Keldeo, Poliwrath, Tentacruel and Jellicent. Mud Shot and Icy Wind both have quite low base powers (55), but have the added bonus of a 100% chance to lower the opponent's speed one stage. Useful if you don't want the defense drop from another Shell Break and if something outspeeds you. Use Mud Shot for type coverage, but Icy Wind could be used if you worry about Tailwind-using Flying-types. Finally, Explosion synergises well with the attack and speed boosts from Shell Break, and could catch an opponent by surprise for a sacrifice KO. Remember that in the 5th generation environment, Explosion no longer temporarily lowers the opponent's defence by two stages, so its base power really is "just" 250.
Cloyster can be countered by pokemon that resist its moves, like those stated above. Tentacruel is easily the best of these, taking at most 174 damage from a +2 Mud Shot with no special defence investment. Jellicent is another Pokemon to watch out for, taking 126 max damage from a +2 Surf, again with no special defence investment. Of course, Cloyster can also be countered by Agility Zapdos, Choice-Scarfed Starmie or Azelf and Agility Kingdra, due to being faster than Cloyster after one Shell Break and having powerful STABed/Super Effective moves.