Using Mako's skill Mythic Depths, you can use WATER monsters to make a beatdown deck by swarming the field and simply relying on their high attack. It's relatively cheap to make since many of them are low rarity or from the card trader.
Hammer Shark's effect lets you special summon a lvl 3 or lower WATER monsters. Atlantean Attack Squad and Spined Gilman are good monsters to summon in a Beatdown deck since they have high attack, but they often work best in pairs instead of by themselves. This is why you need Powerful Rebirth to bring them back. Atlantean Marksman can special summon Attack Squad from your deck when he inflicts battle damage. Since Atlantean Marksman is a Sea-Serpent-type monster Atlantean Attack Squad will gain his attack boost.
Since the monsters in the deck already have some pretty good attack, Hey, Trunade! is good to make sure your attack goes through by returning all your opponent's backrow cards to their hand. Enemy Controller also further helps with some OTK potential by tribute taking your opponent's monster so you can direct attack. These two cards work well together since (if you turn activation confirmation to on) you can go into battle phase and use Enemy Controller to tribute take your opponent's monster, and chain it to your Anti-Magic Arrows so your opponent can't respond with any spell/trap cards.
Use Super Rush Headlong for protection against monsters your opponent summons that you can't go over. It's a quick-play spell so you can use it as protection, or offensively during your turn. This gives you some room for flexibility. Wall of Disruption debuffs your opponent's attack position monster when one of them attacks. This debuff gets stronger the more monsters your opponent controls. This debuff could then help your weaker monsters overpower stronger monsters your opponent uses.
The example deck is used to show a more F2P friendly deck, but an alternative and more expensive version can use these cards to improve on the deck. Cosmic Cyclone is used to snipe dangerous spell/trap cards from your opponent, as well as lowering your LP so you can use Bastion's Silent as WATER skill for an even higher attack increase than Umi. Using Ties of the Brethren lets you swarm the field VERY early, but you will need to remove Hammer Shark for another lvl 3 monster like Lost Blue Breaker.
You will need at least 2 or 3 one or the other to be more consistent, which will bump them from a "somewhat F2P deck" to a "Credit Card deck".
I have almost the same deck setup I use Oyster meister instead for better synergy with hammer shark the only thing this deck is good for is auto dueling SD and roaming LD to get stuff like exp gold keys etc.