You can discard 1 other WATER monster to the Graveyard; Special Summon this card from your hand. When you do: Add 1 Level 4 or lower "Mermail" monster from your Deck to your hand. You can only use this effect of "Mermail Abyssteus" once per turn.
Requires you to have another WATER monster on hand to Special Summon himself.
Abyssteus is basically a free body on the field because his Special Summoning cost can be balanced out with his search.
Here are some monsters you can discard when Special Summoning Mermail Abyssteus to gain additional effects. Atlantean Marksman and Atlantean Heavy Infantry can respectively destroy a face-up card and face-down card your opponent controls. Mermail Abyssgunde and Mermail Abysshilde can respectively Special Summon a Mermail monster from your graveyard and hand
If Mermail Abyssteus searches Mermail Abyssdine from your deck, you can immediately Special Summon that searched Mermail Abyssdine. This puts 2 Special Summoned bodies on your field that you can use as tribute fodder.
Here are some other Mermail monsters you can search with Mermail Abyssteus’ effect.
Use a level-1 Tuner with your Special Summoned Mermail Abyssteus to make a level-8 Synchro monster. If you control no monsters you can first Special Summon Unknown Synchron and then Special Summon your Mermail Abyssteus.