"Hammer Shark", "Spear Shark" and "Double Shark"
"Shark" (シャーク Shāku) is an Archetype of cards used by Reginald Kastle/Nash in the Yu-Gi-Oh! ZEXAL anime and manga. Most of them are Level 3 or 4 WATER-Attribute and/or Fish-Type. Many have swarm effects, allowing easy Summoning of Rank 3-5 monsters. They are only supported as an archetype in the anime and manga.
A few of these monsters, notably "Metabo-Shark" and "Scrap Shark", sport decent to high ATK at low Levels, allowing them to start out as beaters when Summoned. This lets them battle outside the Extra Deck. Their effects, as mentioned, allow them to manipulate their Levels between 3 and 5, allowing them to Summon Rank 3 to 5 Xyz Monsters quite easily.
Seeing as this archetype is built for aggression and swarming, a fast-paced Deck can be built using "Shark Stickers", "Shark Cruiser" (although this card takes a bit more prediction and a lot more luck), and most notably "Hammer Shark" to swarm the field. It is actually quite easy to pull combos with this Deck. The most basic one is to Normal Summon "Hammer Shark", using its effect to Special Summon any Level 3 or lower WATER monster, then Special Summoning "Shark Stickers". "Double-Fin Shark" is an excellent card to make an instant Rank 4 Xyz (though only WATER).
Another rather potent combo is to use the effect of "Bahamut Shark" to Special Summon "Number 47: Nightmare Shark" which can attach another Shark monster like "Shark Stickers" to itself despite being Summoned with no Xyz Materials, and thus gain its rather useful effect too. Since this Deck focuses on swarming the field and being aggressive, cards like "Sea Lord's Amulet" can prevent your WATER monsters from being destroyed by your opponents card effects. Another good combo to maintain field presence is to use "Bahamut Shark's" effect to Special Summon a Rank 3 WATER Xyz monster and then Xyz Summon again to summon "Full Armored Black Ray Lancer" and activate "Sea Lord's Amulet" to prevent "Full Armored Black Ray Lancer" from being destroyed and let "Full Armored Black Ray Lancer" attack your opponent's monsters to destroy his Spell/Trap cards. One should remember that the effect of "Bahamut Shark" only applies to the rest of the turn when restricting its attack. "Saber Shark" can help easily to Summon Rank 5 Xyz monsters like "Number 73: Abyss Splash" and "Number 94: Crystalzero," if one needs to defeat more powerful monsters, while another option is "Shark Fortress," which protects your own monsters and can allow any monster to attack twice during the Battle Phase. "Metabo-Shark" is good due to its effect of bringing back two fish monsters from the graveyard and adding them to your Deck.
One in-Deck "boss monster", introduced in Cosmo Blazer, is "Hyper-Ancient Shark Megalodon" who sports the highest original ATK in the entire archetype (2900). Its effect destroys a monster when it inflicts Battle Damage, further pushing the aggressiveness of the archetype. The true boss monster is seen to be "Number C32: Shark Drake Veiss" whose effect drops an monster's ATK and DEF to 0, while "Number 32: Shark Drake" brings a monster back that it destroyed by battle and lowers its ATK by 1000, while also allowing it to attack again. "Number 47: Nightmare Shark" is also useful due to its effect that allows a WATER monster to attack directly, allowing both a large amount of damage, and the triggering of "Megalodon's" effect. As such, this archetype can be used very aggressively to beat the opponent down. Another technique to summon "Number C32: Shark Drake Veiss," is by using a "Rank-Up-Magic" Spell card on "Number 47: Nightmare Shark," rather than using "Shark Drake" as Xyz Material. Though not Shark monsters themselves, "Number 101: Silent Honor ARK" is an excellent addition for cheap monster removal, with its evolved form being able to be teched in as well, "Number C101: Silent Honor DARK."
Most "Shark" monsters' effects activate only when they are Normal Summoned, so to fully utilize them, you can tech in "Double Summon" or "Ultimate Offering" (though the latter is now Forbidden) to trigger these effects. By doing this, you can Tribute Summon "Hyper-Ancient Shark Megalodon" more easily. "Water Hazard" can be excellent, as the deck can already swarm quite quickly, and can essentially cut down on a turn for Xyz Summoning, as well as expanding your options if you don't have either "Hammer Shark" or "Shark Stickers" in your hand.