Hrmmm
So ice is out...pity.
One thing you'll need to be aware is is that particularly in a subtropical environment, you will likely get mosquitos and algae in stagnant water...at least, if you're being realistic.
You may want some sort of current, probably through a gravity feed system, to keep that down.
There will likely either need to be either a recirculating pump/system (for a closed system prison) or a decanter of everfull water (for an open system prison, which will wind up creating its own watercourse at the drain outlet, particularly if it's been left on for a millenium or two).
Depending on how high-magic and divergent you want to get with the definition of prison, it's entirely possible the spirits were just sealed into glassteel cubes, chucked into the deepest part of the ocean, and checked on periodically with clairvoyance and darkvision or truesight. Maybe some druids with a sea orientation went down to encourage the coral around the cubes to grow faster, plant some toxic dire sea anenomes on top of it, and chum for sharks.