furball: Water should be, and to an extent is, fluidity. All living things have water in them, most environments have water in them, and water is made from the most common chemical substance in the universe, so it's natural that water should have direct synergy with many other elements.
I think Blue Crawler deserves either a moderate raise in HP or a slight raise in both attack and HP. The latter dismantles a synergy with adrenaline, but that isn't prevalent in most metagame, so I think that an attack buff is still best for the card in general.
Laxa: I would remove freeze. While it can be a good card, it is generally overshadowed by most other CC, especially BB. I would replace it with a card that freezes permanents, instead. PC, even only temporary would be a huge boost to water as an element, enough that it would balance the loss of Freeze. Water would still have Arctic Squid, Ice Bolt, and multiple duo synergies built around CC.
As for a deck with implict synergy, I don't really need to build one :3. Both Flowing Reflection and an unupped variant I used in Phase 2, Cascade, utilize it's many duo synergies to form a strong, pseudo-rainbow with no more than four elements.
Poll
