When I was starting to return to running, I frequently did "mixed runs" -- start running on land, then swap into a bathing suit and finish out the miles by pool-running. And even after I started phasing track workouts back in, I would do my warm-up and workout outside, but the cool-down in the pool (conveniently adjacent to the track).
The purpose behind this practice was simply to be careful with the stress I placed on my healing foot, by transitioning gradually from pool to land. But along the way I noted a key side benefit.
I was NEVER sore after a track workout if I followed it with a pool run. Never.
Ditto for races. And also for upper body strength-training. I could do eccentric weight work to failure, and yet the DOMS (delayed onset muscle soreness) would be minimal or non-existent. Again, IF I hopped in the pool right afterwards.
So, I've kept the pool-running as part of my routine even as I've returned to full mileage. Essentially, I make sure to do 20 minutes of easy pool-running after any hard workout or race -- ideally as soon as possible. And the benefits persist.
Here's why I think it works.
a) Cold water. No, it's not as cold as the ideal temperature for an ice bath (supposedly between 54-60 degrees), but apparently it's still cold enough to do the job.
b) Pressure. It's well known that immersion in water results in significant hydrostatic pressure against the body, resulting in a lowered heart rate due to the higher stroke volume that results from the pressure. In plainspeak: more pressure against your circulatory system means that more blood flows with each beat of the heart, so your heart has to beat less often to move the same amount of fluid. This, BTW, is the same theory underlying the use of compression gear during performance.
Now, while the jury's out on the benefits of compression gear while racing, there's a bit more consensus on the benefit of compression for recovery. Additionally, compression is one of the four components of the "RICE" treatment for recent injury (Rest, Ice, Compression, Elevation).
So, connecting the dots, it makes sense to me that the pressure exerted on the body when immersed in water would have a similar recovery effect to that produced by compression gear.
c) Movement. In my humble non-educated opinion, this is what truly separates pool-running from an ice bath. When taking an ice bath, you're most likely seated in your bathtub, meaning your hip flexors (and to a lesser extent, your quads) are in a shortened position. If you have tight hip flexors, as I do, this is an issue. By taking an ice bath while seated, I chill those muscles while they are shortened. In doing so, I undo some of the good work I've put into lengthening those babies and correcting my lordosis/anterior pelvic tilt. But when pool-running, I'm working my legs through their full range of motion, keeping those hip flexors lengthened (and also encouraging blood flow throughout my legs in the process). Much better, I think.
d) More appealing. This one can't be discounted. It's much easier to get into a pool for 20 minutes post race than to make up an ice bath. And the best recovery techniques are the ones you actually use.
So...how do you do it? Well, it's pretty easy. Drive over to the pool after your race or workout, don your bathing suit (showering off the sweat and grime, please), and then hop in.
Once in, I use my normal pool-running form, alternating between cycling and bounding. The key differences are 1) that I really focus on full range of motion, rather than firing muscles, and 2) that I make sure to keep the effort very easy (I recommend using a belt even if you usually don't). The key thing to remember is that this is not a workout, but recovery.