Did you know that? (Dominions trivia)

[vfb]

Most Flying units cannot fly underwater, but the Djinn can.

[Falkor]

vfb said:
Most Flying units cannot fly underwater, but the Djinn can.

Same for Horrors.
Send (lesser) Horror spell is as valuable underwater as on land. The beasts are amphibian and will "fly" directly for horror marked units. You just can't cast it underwater but you can target water provinces.