The strategy of this deck is to destroy your opponent's cards by tributing your own monsters. At first this seems like a 1 for 1 trade, but if done on the right target you can minus your opponent much more than what you are losing making this a powerful control deck.
Hammer Shark is a great card to start your combos. With his effect you can special summon a level 3 or lower WATER monster from your hand. You can use this to special summon Oyster Meister for another beater or Beautunaful Princess to special summon another Fish-type monster from your deck.
If Hammer Shark is how you start your combos beautunaful Princess is how you extend it. When Summoned you can tribute her to special summon another Fish-type monster from your deck. That could be anything in this deck. From a Hammer Shark, Oyster Meister, or Golden Flying Fish. A combo you can use to fill your field is:
Summon Hammer Shark and activate effect to...
Summon Beautunaful Princess and activate effect to...
Special summon another Hammer Shark from your deck...
Activate the effect of the second Hammer Shark to special summon Oyster Meister from your hand.
This is the monster you want to tribute if you want to get additional value from your tributes. When this monster leaves the field other than because of battle you can special summon a token to your side of the field. Meaning if you tribute this monster you can special summon a token, which can also be used as a tribute. Use this monster as tribute for Golden Flying Fish, Enemy Controller, or Fish Depth Charge. Other than as tribute, this card also serves as a decent beater with the Umi field face-up.
Golden Flying Fish is the main control factor in this deck. Tributing another Fish-type monster you control to destroy any card on the field can be great depending on what you tribute and what card you destroy, another advantage is that this effect is not once per turn and can be reused as long as you have monsters to tribute. The best monster to tribute is Oyster Meister because, after you tribute him, he special summons a token which can also be tributed. Meaning you get to destroy 2 of your opponent's cards for the price of 1 of your cards. Another factor is what you destroy with your effect. Destroying an opponent's monster that was difficult to summon (i.e. Ritual monsters, Fusion monsters, tribute summoned monsters, etc.) will net you a lot of advantage. You can also destroy spell/trap cards so that you can attack safely with your monsters.
Powerful Rebith: Revive monsters to use as beaters with an additional attack. Or to use as tribute for your other card effects, in this case Powerful Rebirth will be left on the field.
Parallel Twister: with this card you can get rid of your used Powerful Rebirth on the field and destroy another card on the field.
Enemy Controller: Used to change your opponent's monster to defense position, preventing an attack. Or tribute a monster you control to take control of an opponent's monster. Very useful when facing another deck that uses Fish-type monsters because you can take control of their monster and then tribute it for the effect of Golden Flying Fish.
Fish Depth Charge: Tributes a Fish-type monster you control to destroy a card on the field, plus you get to draw a card. Really good when used with Oyster Meister and Golden Flying Fish to tribute the token.