That one, I find it rather easy, with 4 rules:
Anyone in water, toon or mob, is vulnerable to water spells. Once on land these spells are not effective.
Water spells can be cast only while inside water.
Mobs can also cast water spells.
One is vulnerable to water spells only when he/she/it is in the water.
EDIT: To stick the mobs to the sea bottom only makes sense as we could see the waters are very transparent, so probably not much to eat. I see very well kind of trilobites, sea scorpions, electric mats etc... projecting poisons, darts, electricity, sound etc... It will be pretty nice. But I keep on with the walking on sea bottom with a diving suit... That will be awesome. Btw, the heavy armour helmet is very similar to diving suit.
Anyone in water, toon or mob, is vulnerable to water spells. Once on land these spells are not effective.
Water spells can be cast only while inside water.
Mobs can also cast water spells.
One is vulnerable to water spells only when he/she/it is in the water.
EDIT: To stick the mobs to the sea bottom only makes sense as we could see the waters are very transparent, so probably not much to eat. I see very well kind of trilobites, sea scorpions, electric mats etc... projecting poisons, darts, electricity, sound etc... It will be pretty nice. But I keep on with the walking on sea bottom with a diving suit... That will be awesome. Btw, the heavy armour helmet is very similar to diving suit.