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 - Gaioabyss", 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, "Abysstrom," uses "Umi" for its effect. This is also evident in the new Field Spell introduced in their debut set of Abyss Rising (which, namewise, partially refers to them as well) whose effect treats it as "Umi." The Field Spell, Forgotten Capital Lemuria, helps them Xyz for higher Rank Xyz Monsters, and partially supports the summoning of "Gaioabyss". 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 genrally 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 9 ("Number 9: Canopy Star - Dyson Sphere"). With 2 "Megaloabyss" and "Forgotten Capital Lemuria") they can make Rank 9 fairly easily. Rank 4 is accessible with just the Level 4 monsters themselves. Rank 5 is created with Forgotten Capital Lemuria and 2 Level 3 monsters, while 6 is reached with 2 Level 4 monsters and Lemuria. Rank 7 is reached plainly by 2 "Mermail - Megaloabyss" without help from Lemuria. Rank 8 is impossible without 3 Level 5 monsters or 4 Level 4 monsters, both 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 oponnent monsters with Mermail - Gaioabyss, using his effect to negate the effect of face-up enemy monsters, as his effect can be activaded in any player's turn. To summon him, one can Special Summon Mermail - Megaloabyss's own effect, and then adding Abyssphere to your hand, letting you summon another Megaloabyss in your next turn. This can also be done with Mermail - Abysslinde's effect to summon Megaloabyss from your deck. Using the effect of Forgotten Capital 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.
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 Armed Soldier," "Atlantean Marksman," or "Atlantean Dragoons, to activate the Mermails effects you can destroy a card on the field (Armed Soldier and Marksman) or search your deck for any Sea Serpent type monster (Dragon Riders), such as Megaloabyss, while also gaining the Mermail's effects.