Water terrain - each Upkeep phase all living non flying non mage creatures get a drown token (without X trait, like aquatic) in the water terrain, whenever a creature has 3 drown tokens, destroy it immediately. Immediately discard drown tokens if a creature is not in water terrain.
Siren could have spells like - suffocate, incantation, gives out extra drown tokens or even mage's ability, pay 1 mana to give drown token once per turn.
Water prison - incapacitated, drowning, u can remove the bubble and free the creature by attacking it and dealing damage. Enchanment or mage's ability - cantrip conjuration, which could be cast as a free action (paying mana cost, 2X creature level) after water based spell attack.