First of all, if you want to play this deck as a budget version, cut out the legendaries you don't have, and put in Darkspear Berserker, or  Felfin Navigator. 

Now on to the guide:

MULLIGANS: 

I will rate the cards on a 4-point scale with 1 meaning you almost always keep it, and 4 meaning you almost never keep it.

Lightning Bloom (2). This card depends on whether you have something to ramp into.

Murloc Tinyfin (1). This card is a good play on one if your opponent doesn't have a "deal one" heropower (warrior, paladin, priest, etc.), but is usually played on two with your other murlocs. Also works with Nofin Can Stop Us, or Coldlight Seer. 

Murloc Tidecaller (1). Really good in almost every matchup, and can be a 1 mana 4/2 or better. It is bad against Sword and Board in warrior, and Backstab in rogue, so you have to consider that, but usually you keep it anyways.

Spawnpool Forager (1). It is the best way to ensure you have a turn two murloc activate, and is sticky against clears.

Cagematch Custodian (2). Just a generally good card in shaman, curves right into the Whack-a-Gnoll Hammer. Good against aggro or midrange matchups. Bad with a lot of the murloc synergy though.

Firemancer Flurgl (1). This card can devastate aggro decks when coined out. A must-remove in control matchups as well, so try and keep cheap murlocs to follow it up.

Lushwater Murcenary(2). This card is good stats, but is bad when you don't have a turn one murloc. 

Lushwater Scout (3). Work well when going second with Murloc Tinyfin, as the tinyfin trades into their two-drop, and then the 1/1 gets rush. Works alright against aggro or tempo as long as you have a way to keep it around. 

Murloc Tidehunter (2). Curves really well into the 3 drop murloc synergy of Magicfin, Coldlight Seer, and Nofin Can Stop Us.

South Coast Chieftain (3). This card is good against aggro, or some midrange decks, but is used to finish control decks. Don't keep against priest, warrior, or rogue (assuming this miracle rogue thing gets popular). 

Wriggling Horror (3). Unless you have a one-drop and murloc tinyfin, you'd almost always want a different 2 drop. That said, it isn't horrible to play on two as long you can buff at least one creature. 

Coldlight Seer(3). Good to curve into, but generally is too inconsistent to keep, as if you have your early plays removed, it is just a 2 mana 2/3. Do not keep if you are keeping Cagematch Custodian.

Instructor Fireheart(3). Usually you hope to draw this late in the game, and find a ton of burn spells off her ability. Not awful when played on 3, just generally better saved for later. 

Magicfin (2). Works against aggro as you can trade into their minions. Good to play on three against control to prevent coin-Holy Nova or turn 4 holy nova (or other 2 damage aoes). Don't keep with Cagematch Custodian.

Nofin Can Stop Us (2). Very strong against control, and alright it almost every other matchup. It does not work well with Cagematch Custodian.

Whack-A-Gnoll Hammer (3). You should keep this card against tempo and aggro, and otherwise toss it. Whack-A-Gnoll Hammeris mainly in because it is good for curving out with Cagematch Custodian and is strong against aggro in general.

Kazakus, Golem Shaper (4). You want to play this on 4, so you can only keep it when you have a 1-2-3 curve. Make sure your curve is flexible if you keep this. 

Inara Stormcrash (3). Used to finish off control decks, and if ramped to early with Lightning Bloom she can end the game if the opponent doesn't have hard removal. Don't keep without ramp.

Deck Strategy:

The idea of Murloc Shaman is to play some cheap murlocs and curve into Nofin Can Stop Us and get a huge board. You play Kazakus on 4, going into 5-cost golem. You can also win by getting out a quick Inara Stormcrash. Late game, Instructor Fireheart can burn down the opponent and can provide a lot of value. Magicfin can also win the game with all the random legendaries.

I will update this section with more info once I playtest this deck some more. I will also give more class-matchup advice. I also will take advice in the comments if you have anything to say.

Card Considerations:

SPELL SHAMAN

More spells can be added to this deck to make it a more midrangey style. Serpentshrine Portal, Diligent Notetaker, Novice Zapper, Stormstrike, Torrent, and Marshspawn. You can try adding a few of these, but if you add too many then it dilutes the deck and you might as well just play regular ol' face shaman.