Curse of the Depths

You curse your target to be at the mercy of the depths of the ocean. The target must make a Charisma saving throw. On a failed save the target's lungs begin to fill with water and they suffer the oppressing weight of the ocean. For the duration the target cannot cast spells that require verbal components and at the beginning of each of their turns they suffer 1d6 suffocation damage. Any creature that does not need to breathe is immune to this damage. On a successful save nothing happens. The target can make another Charisma saving throw against the spell's effects at the end of each of their turns, ending the spell on a success.

At higher levels: 
When casting this spell at 5th level, the target's speed is reduced by half and the duration of the spell is increased to 1 hour.
At 7th level the target only makes the saving throw once, when the spell is cast, and the duration of the spell is increased to 8 hours. If the target fails the saving throw the spell can only be ended by a remove curse spell of 7th level or higher or a wish spell.
At 9th level the curse lasts indefinitely, deals damage even to creatures that don't need to breathe, and can only be removed by a wish spell.