Whirlpool
Ahead the waves break in strange ways and suddenly the current moves in a circular direction with unlikely swiftness. This stream within the water increases in speed and force, drawing towards a center where it all pitches downward into a maelstrom that drags everything below the surface!
Realize Danger. With a Nature check an adventurer spots the furthest reaches of the vortex and can point out that within 100 feet of it the waters are dangerous, granting advantage on checks made against the vortex while outside of its area.
Spell Solve. A casting of control water is not large enough to affect the entire whirlpool, but creatures within the spell’s area gain advantage on checks made against the whirlpool.
Vortex. The center of the whirlpool is a churning vortex with a 500-foot radius area on the surface of the water. Creatures and objects that are in the vortex’s area or within 100 feet of it make an Athletics check (or a water vehicle check if onboard a vessel), and on a failure are pulled 20 feet towards the area’s center. Once within the area, checks made to escape it have disadvantage .
When a creature first enters the vortex’s area on a turn or starts its turn there, it makes a Strength saving throw , taking 9 (2d8) bludgeoning damage and being pulled towards the center of the area on a failed save. On a successful save, the creature takes half damage and isn’t pulled.
If a creature or object fails on an ability check or saving throw against the whirlpool while within the center of the vortex’s area, it gets sucked down 50 feet (1d10 × 5) feet below the water. After a creature or object is sucked down 60 feet or more under the water, the vortex moves it 260 (5d20 × 5) feet in a random direction.
Water Solver. Creatures with a swim speed gain an expertise die on checks made to escape the vortex.
A group water vehicle check can be made to navigate a vessel around and through the whirlpool. Sufficiently large debris (such as a Huge-sized or larger object) can temporarily plug up the whirlpool, suspending the effects of the vortex for 2d4+2 rounds.
Critical Failure: The adventurers are sucked into the whirlpool’s vortex, and each suffers 1d4 levels of fatigue and 1d4 levels of strife by the time they escape its grasp.
Failure: Each adventurer suffers a level of fatigue and strife but they manage to avoid the worst of the whirlpool’s wrath.
Success: The adventurers skirt the edges of the whirlpool without too much trouble.
Critical Success: The adventurers ride upon the countercurrents created by the whirlpool, riding the waves to gain incredible momentum that reduces their travel time through the region by 1 hour.