I'm currently working on a superhero story where a speedster (someone with the superpower of running really fast) loses his speed while moving at max speed: Mach 2. This loss happens without him being able to slow down, essentially pulling him from Mach 2 to 0 in an instant. One of the abilities that falls under his powers is immunity to G-forces from acceleration and deceleration. This loss happens while he's running out on the ocean. I imagine this is bad for him, but just how bad? And does some kind of shockwave hit the water once this loss hits him?
Instant stop = reduced to pulp.
Of course, it's more likely your speedster would still be going at Mach 2, but suddenly unable to move his legs fast enough to maintain that speed against air resistance.
With the water passing underneath at Mach 2, his next step will likely result in a torn-off foot and falling forward. The Mach 2 airflow from the front will then lift him, cause him to spin uncontrollably for a while, perhaps tear off clothing, hair and/or limbs. All that doesn't matter as the hero has died from massive hemorrhaging in his brain due to the G-forces involved in the spin.
Then he crashes into the water and turns into a cloud of fish food. There might be a small shock wave involved, but no more than he was already causing running over the water at Mach 2... Some animals involved in this production will have been harmed.
Make the power shut down over a short period of time with some kind of hysteresis, like an inductor that loses its charge. You can arrange it to suit the narrative, losing active ability to run and then diminishing protection from his existing speed. The protection can diminish a little faster than his speed, so he suffers a crash that he feels but doesn’t injure him beyond what your plot needs.