In looking at the question Can the Erupting Earth spell be cast somewhere that isn't on “ground”?, other examples of spells involving the "ground" could be useful. Or not. Which leads to the question:
Where does a creature - flying low over a large body of water - descend when subject to the Earthbind spell?
According to the description of Earthbind (XGtE, p. 155):
An airborne creature affected by this spell safely descends at 60 feet per round until it reaches the ground or the spell ends.
Assuming the creature began its turn 60 feet above the water and failed its Strength saving throw, which of the following would happen?
- Would the spell end when the creature reaches the water's surface after 1 round? (In other words, is the surface of the water "ground"?)
- Would the creature continue to descend for the remaining 9 rounds for as much as 540 feet to the earthen bottom of the body of water (see note below)?
- Would the spell fail altogether?
- Would the creature be forced essentially sideways for the duration of the spell, towards the nearest point of land?
- Would something else happen?
Note regarding being forced towards bottom of large body of water:
Given a creature with 10 Constitution, 30 ft movement, and no innate swimming speed, its movement in the ocean would be 30 ft using both its Movement and Action to "Dash" (or 20 ft in difficult terrain).
540 ft of total movement would take 18 rounds (27 rounds in difficult terrain) and the creature - assuming it can't breathe underwater or teleport in some fashion - would be unable to hold its breath after 10 rounds and would drop to 0 hit points after the next round.
'cause if the surface of the ocean is ground, guess what might... erupt?