Adding on to Trachy's answer:
Another viable item for Shell Smash Cloyster, especially in formats with less powerful Pokemon, is White Herb, as the neutral Defenses can help against Pokemon if they can't immediately threaten Cloyster (or the format has a dearth of hazard removal options, which renders Focus Sash useless.)
Liquidation is now the preferred Water STAB due to its higher power.
Another option for Cloyster is Ice Shard. Even at maximum speed (which is doesn't usually run over maximum attack) it stil struggles to outspeed Booster Energy Iron Valiant, Scarf Darkrai, Scarf Meowscarada, and a whole host of Pokemon. Ice Shard allows Cloyster to at least deal some damage before fainting (or who knows, you might get a crit) along with providing excellent revenge killing against the likes of Dragon Dance Dragonite (with Multiscale broken.) In lower tiers with less speed, though, the standard trio of Icicle Spear, Liquidation, and Rock Blast is preferred.
