This Mermail deck is an OTK variant with a good ability to gain advantage. Use the Atlantean monsters and Abyss Soldier to clear a path for Abyssmegalo and other monsters to attack safely. Mythic Depths gives a small attack boost to all your monsters which could be helpful for ending the duel.
A Boss monster you can summon with relative ease, he is the main focus of your deck and your main damage dealer when performing OTKs. Summon Abyssmegalo from your hand by discarding 2 WATER monsters to gain field presence. You can then search your deck for an Abyss card to support your Mermail monsters depending on the situation. As an OTK enabler you can tribute one other WATER monster on the field to have Abyssmegalo attack twice during the battle phase, which could be deadly with a base 2500 attack and can be boosted by the Abyss card you search and Umi on the field.
Both of these monsters discard WATER type monsters from your hand as cost for their effects. Mermail Abysspike is used to search out level 3 WATER monsters from your deck. Abyss Soldier returns a card on your opponent's field back to hand. These effect will be used to discard Atlantean monsters or Mermail Abyssgunde to gain advantage.
Atlantean monsters will primarily be used as effect costs for your Mermail monsters. Use discard them to special summon Abyssmegalo, activate the effects of Abysspike and Abyss Soldier, or tribute them on the field to allow Abyssmegalo to attack twice. When sent to the graveyard as cost to activate a WATER monster's effect Atlantean Marksman targets a set card your opponent controls and destroys it. Atlantean Heavy Infantry has a similar effect but for face-up cards instead. Keep in mind that your the monster you target must remain in position (face-up or face-down) for the destruction to go through. If ever the target's position changes it would not be counted as the same target and therefore not destroyed.
When discarded Mermail Abyssgunde revives a Mermail monster in your graveyard except herself. She synergizes very well with Abyssmegalo, Abysspike, and Abyss Soldier in that they are able to discard her from your hand, and they are also able to put Abyssgunde's revive target in your graveyard.
Abyss-scales are equip cards that give your Mermail monster an attack boost and effect negation at the cost of sending the equip card to the graveyard, Mizuchi negates Spells while Cetus Negates Traps. These cards can be searched when you special summon Abyssmegalo with his own effect. Equip them to Abyssmegalo to for protection when trying to perform an OTK.
Can be used as a substitute for Abyssmegalo, but more costly to summon. Can knock a card out of your opponent's hand for disruption, but has less OTK potential than Abyssmegalo.
Silent As WATER activates Umiiruka which gives your monsters a larger attack boost than Umi, but requires your LP to drop below 3000 to use unlike Mythic Depths which activates immediately at the start of the duel.
Silent as WATER Can be used after your Life Points are at 3000 or below. Play the Field Spell "Umiiruka." If there already is Field Spell on your side of the Field, rerturn it to your hand. This skill can only be used once per Duel.