One of my players recently got access to the control water spell, and in the last session, had the opportunity to use the Whirlpool option for the first time. It went badly! This spell is so poorly written that it brought the game to a screeching halt while the player and I tried to figure out what exactly the whirlpool does.
If you're interested in our problems, consider the following questions: Under what circumstances does a creature need to roll an Athletics check to 'swim away', other than using an action to stop being trapped in the vortex? What does it mean for a creature to be caught in the vortex "until the spell ends" but then make a save at the start of each of its turns where success means it "isn't caught"? What is the game effect of being "caught"? I also had difficulty adjudicating the shape of the whirlpool on a map. (I'm not particularly interested in excessive discussion of the answers to these questions; even if I make a ruling, I feel the spell is badly written enough that it will continue to cause problems and confusion.)
I made a ruling just to get through that combat, but afterward my player voiced concerns about using this spell in the future, both that the mechanics were convoluted and that the effect was ultimately unimpressive given that it costs a 4th level spell slot. While it seemed like a good choice for an adventure that features many underwater foes, it was a disappointing result.
So that's what prompted to examine this spell and try to come up with a homebrewed version that is quick to apply at the table and has an effect that's worth the cost of casting. I'm hoping to get an idea of whether my proposed rewrite is a viable replacement for the Whirlpool option of control water, while changing nothing else about the spell.
Whirlpool. A vortex forms in a cylinder 25 feet tall with a radius of up to 25 feet, centered on a point you choose within range. This effect requires a body of water large enough to contain the entire cylinder. The vortex is difficult terrain, and is surrounded by a zone of suction that extends a further 25 feet.
When a creature starts its turn in the water within the vortex or the zone of suction, it must make a Strength saving throw (or choose to automatically fail). On a failure, it is pulled 10 feet towards the center of the vortex (provided there is an unoccupied space to do so) and becomes unable to move further away from it until the start of its next turn. Then, if the creature is within the vortex, it takes 2d8 bludgeoning damage if it failed the save, or half as much if it succeeded.
At the start of your turn, unattended objects that are not secured are moved and damaged as if they had failed the saving throw.
As an action, a creature that failed the saving throw can make a Strength (Athletics) check against your spell save DC. A creature in the vortex takes disadvantage on this check. On a success, the creature can move away from the vortex.
I think this compares favorably to other spells of the similar level, like Evard's black tentacles, sleet storm, and ice storm, with a huge area and moderate crowd control. Does this seem balanced? Are there any problems with the wording or rules interactions?