Mulligans
General Mulligans
Generally, you want your draw and cards that activate your Jan'Alai. But depending on the matchup, you can alter your mulligan to have Malacrass give you some extra value.
Aggro Mulligans
Use your Ice Walker the turn after you spirit in order to freeze 3 targets. Keysmith into Ice Barrier and Runes, Pyromaniac has a solid body and Shimmering Tempest/Ghastly Conjurer trade up and down respectively. If Pyromaniac can survive a turn then you can fire eater spirit ping to kill 3 2 or less health minions and draw 3 cards, which is some nice value.
EDIT: Now that I know that Ice Walker doesn't actually synergize with the spirit (something about it saying "the" target not working with multiple targets) I switched it out for two Waterboys, which are a free 2/1 effectively and you can ping something so it's basically a blowgill without murloc synergy.
Control Mulligans
Against control, you just want to survive until the late game. Keep a Spirit and Fire-Eater on hand so you can summon more water elementals. Use Jan'Alai conservatively but consistently to generate value, as with Malacrass.
Use Arugal and Leyline to create many minions. A well-placed Fire-Eater Dragonhawk Ping on turn 5 will activate Jan’Alai. You can play Jan’Alai on turn 7, or you could wait until turn 9 and bounce it with Brewmaster or Banker or add a golden copy of Jan’Alai with Zola on turn 10. Use Malacrass to strategically add cards you may need to your hand, then Banker it. Use Jaina to get insane value out of Fire Eater Spirit HP summoning 3 water elementals and casting a spell damage +2 shooting star- all for 5 mana! Ping your acolytes for draw. This is going to be a good, flexible deck.