(Best) Ultimate Water Deck

This water deck speacializes in getting heavy-hitters into play, and quickly getting the engery required to power them.

You will want to play Kyogre EX first (or find him first). Utilize Kyogre EX's Water Pulse to stall the oppont's active pokemon while you locate your other heavy-hitters, such as Walrein, Avalugg, and Wailord V. Keep in mind that Wailord V is best played after your opponent has knocked out Kyogre EX and/or other pokemon, as Wailord V's Draw Up can quickly retrieve the knocked out

This water deck speacializes in getting heavy-hitters into play, and quickly getting the engery required to power them.

You will want to play Kyogre EX first (or find him first). Utilize Kyogre EX's Water Pulse to stall the oppont's active pokemon while you locate your other heavy-hitters, such as Walrein, Avalugg, and Wailord V. Keep in mind that Wailord V is best played after your opponent has knocked out Kyogre EX and/or other pokemon, as Wailord V's Draw Up can quickly retrieve the knocked out pokemon's lost energy and power himself up quickly.

Lapras GX and Tapu Fini GX are great for futher hindering your opponent if Wailord V, Kyogre EX, and your other heavy-hitters are proving hard to find. Use Lapras GX's Collect to reveal more cards in hopes of finding Wailord V or another strong pokemon. Use Tapu Fini GX's Hydro Shot to eliminate your opponet's most threating pokemon, whether they be active or on the bench (target Electric type pokemon as they pose the largest threat to Wailord V). You can also use Tapu Storm to at least tempoarily remove any threat, especially if the threatening pokemon has more than 120 HP and thus cannot be removed with Hydro Shot. Finally, once Wailord V is found and played to the bench, use Tapu Fini GX's Aqua Ring to quickly replace Tapu Fini GX with Wailord V as your active pokemon. (Another great way to get Wailord V is the Trainer items Pokemon Communication and Great Ball)

There are a few other methods to slow down your opponent, and I will only breifly mention them: Drednaw's Ability, Exoskeleton; and Walrein's attack, Hail Prison.

Notice that a few pokemon, such and Tapu Fini GX and Walrein discard energy from themselves as an effect of their attacks. Don't hesitate to use these moves prior to playing Wailord V (just not too much), as Wailord V greatly benefits from having water energy in your discard pile as long as you have one water energy to use the move Draw Up. Also note that enemy actions to remove energy from Wailord V are quite pointless as he can pull such large amounts of energy (3) from your discard pile.

You will surely surprise your opponent when you use Max Revive to bring back Wailord V after he has been knocked out once. Using the Max Revive on Wailord V is extra oppressive for your opponent as Wailord V was likely knocked out with a lot of water energy attached to him, all of which would be presently in the discard pile. Therefore, Wailord can quite easily get back all the energy in the discard pile that was attached to him by using Draw Up.

To power up your other pokemon, utilize Lady, Energy Retreival, Professor's Letter, etc. 

Finally, I want point out that, although this deck is made up of only water type pokemon, there are a variety of weaknesses present: Electric (to be expected), Steel, and Grass. Because of this, one pokemon type won't be able shut down your entire deck.

Best of luck!