Level 3 "Mermail" monsters all share a similar effect when they get sent to the Graveyard - they Special Summon another "Mermail", letting them swarm the field. The Level 4 "Mermail" monsters can activate their effects when they are Normal or Special Summoned, by discarding a WATER monster from your Hand, which will trigger the effect of the previous monsters effects, and the Level 4 monsters help to either search the Deck or Graveyard for a Level 3 WATER monster. The higher-Level "Mermails" can Special Summon themselves with relative-ease by sending other "Mermails" to the Graveyard, which will also trigger their effects. The "boss" monster of the archetype, "Mermail Abyssgaios", is a powerful control card, with 2 effects that forces your opponent into a corner, either by preventing their most powerful monsters from attacking and/or by negating their weaker monsters' effects.
They have a small relation to the "Umi" series, since one of their support cards, "Abyss-strom", uses "Umi" for its effect. This is also evident in the new Field Spell introduced in their debut set of Abyss Rising (which, name wise, partially refers to them as well) whose effect treats it as "Umi." The Field Spell, "Lemuria, the Forgotten City", helps them Xyz for higher RankXyz Monsters, and partially supports the Summoning of "Abyssgaios". Combining the original "Umi" cards with these newer cards is not a good option, since there are much better options available for this task which outclass the "Umi" cards. The "Umi" cards only have 1 connection in which they function properly, and their effects are generally weaker than those of the "Mermail". Another possibility for a field spell is "Clear World". It causes the user of a water monster to discard at the end of their phase, facilitating their effects, while possibly hindering the other player.
Swarming and Xyz Summon is thus their main style of play, and with all the recent support for WATER-attribute monsters, can do a decent job of Summoning Xyz Monsters. By utilizing support they can reach from Rank 3 ("Number 17: Leviathan Dragon" and "Black Ray Lancer") to Rank 9 ("Number 9: Dyson Sphere"). With 2 "Mermail Abyssmegalo" and "Lemuria" they can make Rank 9 fairly easily. Rank 4 is accessible with just the Level 4 monsters themselves. Rank 5 is created with "Lemuria" and 2 Level 3 monsters, while 6 is reached with 2 Level 4 monsters and Lemuria. Rank 7 is reached plainly by 2 "Abyssmegalo" without help from "Lemuria". Rank 8 is impossible without 3 Level 5 monsters, 4 Level 4 monsters or 5 Level 3 Monsters, all of which seem like a waste of cards to spend on what's currently available at this Rank.
Another strategy is to lock the attacks and effects of strong opponent monsters with "Abyssgaios", using his effect to negate the effect of face-up enemy monsters, as his effect can be activated in any player's turn. To Summon him, one can Special Summon "Abyssmegalo's" own effect, and then adding "Abyss-sphere" to your hand, letting you Summon another "Abyssmegalo" in your next turn. This can also be done with "Mermail Abysslinde's" effect to Summon "Abyssmegalo" from your Deck. Using the effect of "Lemuria" and the swarming power of "Mermails" can help as well, needing 3 face-up Level 4 "Mermails" or 4 face-up Level 3 "Mermails".
"Abysslinde" is one of the Deck's most important Summoning cards. It acts as a "Mother Grizzly" for "Mermails" that also works upon being destroyed by card effects.
"Mermails" also go hand-in-hand with the "Atlantean" archetype since their effects are activated when sent to the graveyard to activate a WATER monsters effect. By discarding "Atlantean Heavy Infantry", "Atlantean Marksman" or "Atlantean Dragoons", to activate the "Mermails" effects you can destroy a card on the field ("Heavy Infantry" and "Marksman") or search your Deck for any Sea Serpent-Type monster ("Dragoons"), such as "Abyssmegalo", while also gaining the "Mermail's" effects.
The "Mermails" used to peacefully live in their underwater city, "Lemuria", and albeit good in the arts of fighting, they would rarely use this ability. However, sudden changes in the earth's crust made "Lemuria" rise above the ocean, leaving the "Mermails" homeless. In that moment, the Emperor of the "Atlantean" army, "Poseidra", choose to let the "Mermails" live under his protection, but they would have to obey his orders. Using the great treasure of the "Mermails", the Golden Ring, to control the weather, "Poseidra" ordered an all-out attack against their archenemies, the "Fire Kings".
"Thunder King Rai-Oh" and "Mistake" severely slows down this Deck theme as it halts all of the searching effects of "Mermail" monsters while also being a higher ATK than most of them as well. "Deck Lockdown" also works and stops Special Summoning from the Main Deck too, but can be more easily countered so it is more useful as an early disruption tactic and for Tribute-themed and/or Synchro/XYZ monster focused Decks that have no need to Special Summon from the Main Deck at all.
"Debunk", "Forbidden Graveyard", "Necrovalley", "The End of Anubis" and "Soul Drain" also halts many of the key monster effects of a "Mermail" Deck as well to slow it down, but it does not affect the Xyz Summon options available unlike "Thunder King Rai-Oh". "Skill Drain" also affects the speed of this Deck and can assist in slowing it down too, but without "Debunk" or "Soul Drain" (or other similar Graveyard negation effect cards) their Graveyard activating effects are not affected and can result in its destruction.
One major problem a "Mermail" user might face is "over-extending". Because so many "Mermail" effects rely on discarding or being discarded, a Duelist using the archetype might spend all but a couple of cards in their hand on swarming the field with monsters. Despite the many searchers of the archetype, a player might exhaust all of the options they built with a large hand and fall victim to a well-timed "Gorz the Emissary of Darkness".