Ok first off let me tell you about the sea serpent type, it is one of the most unsupported types out there but this is now changing and is now leaving...ok fine there still the most unsupported type out there(divine-beast does not count) well anyways onto the guide we go.
Monsters
You can Special Summon this card (from your hand or Graveyard) by Tributing 3 Level 3 or lower WATER monsters you control. When this card is Special Summoned by this way: Return all Spell/Trap Cards on the field to the hand. If 3 or more cards are returned this way, all monsters your opponent controls lose 300 ATK for each card returned by this effect.
Sea Emperor Dragon Poseidra
ok at first sight you may be asking yourself "how on earth am I even going to summon this thing" well we will be getting into that later on in the guide
run 1-2
While this card is face-up on the field, once per turn during your Main Phase, you can Normal Summon a Level 4 or lower Sea Serpent-Type monster in addition to your Normal Summon or Set. (You can only gain this effect once per turn.) When this card is sent to the Graveyard for the activation of a WATER monster's effect: Destroy 1 face-up card your opponent controls.
Armed Soldier of the Sea Emperor
ok here is one of the new cards and one of the best sea serpent cards there are this is a level 2 sea serpent water monster.
run 3 always
this is our tour guide and is also one of the two key cards for summoning out Sea Emperor Dragon Poseidra the other one being Armed Soldier of the Sea Emperor
run 3
Assault Troops of the Sea Emperor
If you control another face-up Fish, Sea Serpent, or Aqua-Type monster, this card gains 800 ATK.
His effect is nice because it turns him into a 2200 atk beatstick and he also level 3
run 2-3
now heres somthing I beat you all had no idea was, so while it may not be a sea serpent it still goes great with the deck, this card can also stop the wind-up loop and dino rabbits but I will let you figure out how to do that on your own
run 1-2
because this deck revolves heavyly around the grave this is a very nice card to have just remember you have to have a genex controler in your deck before you can dump a water monster.
run 2-3 run 0 in pure water builds
spells
this card helps alot because it lets you recover nearly any card in this deck that is not dragon ice or Sea Emperor Dragon Poseidra but those two you want to stay in the grave.
run 2-3
ok well I am done for tonight I will be updateing this guide as more cards are released that help the sea serpent type.