A magic-user wishing to avail themselves of this spell would be advised to cast it near a large body of water – as soon as the last syllable of its incantation is uttered, the caster’s form is altered to survive in an aquatic environment. The spell lasts for 1 hour per caster level.

The exact appearance of this new body is left up to the caster. No matter the shape they choose, for the duration of the spell they will gain ability to breathe and move unimpeded underwater. The caster will also become resistant to extreme cold and atmospheric pressure and suffer no impairment to their vision while submerged.

The caster also chooses one of the following enhancements:

1. A natural melee attack (bite, claw, barbed tentacle, etc.) doing 1d6 damage.
2. Chromatophoric skin that grants advantage on Stealth checks underwater.
3. The ability to communicate with aquatic creatures (as the spell speak with animals).
4. Cartilaginous body that can squeeze into or through spaces half as large as normal.
5. Sensory organs that grant blindsight, 60 ft.
6. A siphon-jet that expels water with great force (as the spell gust of wind).

However, the modifications to their body means that the caster cannot breathe air nor speak. When the spell ends, the caster immediately returns to their natural form (hopefully within sight of dry land). The caster will also change back if reduced to 0 hit points while transformed.

Miscast results (1d12):

1 – Continually vomit sea water whenever the subject tries to speak for the duration of spell.

2 – Lose ability to breathe air, follow rules for characters holding their breath

3 – Spell works too well – the caster is transformed into a fish (or some other harmless aquatic creature) for the duration of spell.

4 – Spell effects are permanent.

5 – X cubic feet of air around caster is replaced with sea water, where “X” equals the caster’s level.

6 – Summon a sea monster; it has hit dice equal to the caster’s level and all the abilities granted by the spell, including all six enhancements. It’s probably not pleased with being bothered by a land-shrimp like you.

7+ – Consult standard miscast table.

Inspired by the James Raggi method of writing new spells, which I first read about here, and has more recently been described in Vaginas are Magic!, the 2017 Free RPG Day offering from Lamentations of the Flame Princess. It’s currently available on RPGNow for pay-what-you-want, so you have very little to lose and potentially a lot to gain.