Primordial Aquatic Control Shaman

General Mulligans

The best General curve for the deck, unless you manage to Discvoer Lady Vashj and play her instead of Nimbus on turn 2. Turn 3 Serpentshire Portal will deal with your opponent's minion and summon one for you as well as overload you. That wont be a problem because you've got Marshspawn with which you'll adapt to the current game state AND fulfill the requirement for Lilypad Lurker on turn 5!

Mulligan Torrent with Primordial Studies when playing vs Paladin to easily deal with Crabrider on turn 2.

Chain Lightning is also not bad mulligan but depends on the matchup.