Hold your breath...
Waterborne is a sigil that allows the creature to be submerged for the enemy's turn, causing the opposing creature to skip over it when attacking. The only consistent ways to kill a Waterborne creature are either by doing overkill damage to it before it spawns or by getting it to attack a creature with Sharp Quills.
Kraken Waterborne[]
For tips on using this variant of the Waterborne sigil, see Great Kraken, Hand Tentacle, Bell Tentacle and Mirror Tentacle.
A special version of this sigil can be found on the Great Kraken. Though it misleadingly possesses the same description, it differs in that, when a card bearing this sigil re-emerges, it transforms into a random Tentacle card, losing this sigil in the process. If the Great Kraken is sacrificed onto another card, then the original cards Sigils will also be lost.
Strategy[]
A card bearing this sigil submerges itself during its opponent's turn. While submerged, opposing creatures attack its owner directly.
In Battle[]
Depending on the scenario, cards with this sigil can be placed opposing high or low Power creatures to great benefit. High Power creatures can be defeated without risk of the attacking Waterborne creature dying, though the opposing creature may simply defeat the player before it perishes. Alternatively, a creature with low Power can be defeated without dealing much direct damage (especially useful if said creature possesses Touch of Death).
Damage trading should be considered when playing a card with Waterborne. For instance, if a River Otter is played against a Grizzly, the Grizzly will simply soak meager damage from the Otter and deal 4 damage effectively unimpeded. The player can also use this against an opponent to deal consistent direct damage.
Playing a Wolf (with 3 Power) against a Kingfisher (with 1 Power) will effectively result in dealing a constant 2 damage over time, with no risk to the Wolf as the Kingfisher is also Airborne.
As previously mentioned, Sharp Quills is the only consistent way to defeat a submerged creature, though they can often just be blocked by high Health cards. Alternatively, most cards that bear this sigil naturally have only 1 Power, so Stinky can be used to neutralize them.
When The Trader plays a card with the Waterborne sigil you can still attack it since it hasn't submerged yet.
Deckbuilding[]
This sigil is well-placed on vulnerable high offense cards (Mantis God and Adder for their low Health, Strange Larva for needing to survive two turns to become Mothman, etc.), as this sigil will protect them from being immediately counter-attacked and killed. Conversely, this sigil has little utility on cards with high Health, since this sigil prevents them from being able to block attacks. Since these cards are difficult to kill, Many Lives and Fledgling can be applied without risk of the card dying before they can be properly utilized. Waterborne makes the utility of Unkillable irrelevant and adds no benefit towards the sigil. In Kaycee's Mod, when the Squirrel Fish Challenge is chosen, all squirrels in the side deck gain Waterborne, which is useless, as the squirrel has no attack power, and adding Waterborne simply removes its capability to block powerful attacks.