Silly question to me; but I realize that I am most jaded on this topic. I know that I am pushing an uphill battle when it comes to changing perspectives on how water training could be used....but that's okay. I am up for the challenge.
For me, I never questioned if water training would work, I just tried it. I didn't have a choice. I was in my late 20's and my body started screaming at me when I cranked up the training for Western States (mostly because of tendonitis). My body just couldn't handle the land based volume necessary to partake in ultra distances of that magnitude, without breaking down. If you've read my book, you know what I did. I hit the pool.
My water training consisted of strokes I learned through Shirley Archer and Sharon Svensson as well as using a kickboard several miles each week. This was in addition to weight training and land based running days, especially the LSD day. It worked. I completed a race that 30-50% of the field (depending on the year) doesn't.
However, I know that even after telling my story I am still asked "will it really work?" I am so practiced at water training, that the question arrives to my ears as oddly as if asked "if a plane can really fly." Um, yes, it can and yes it does.
Let me explain how it works.
Consider your own running pace and then think about your training efforts. Some days it is a tempo run, others it is your steady state. Well, water is the same. In my book (page 85) I utilize David Brennans excellent rate of perceived exertion (RPE) scale comparing water running with land running, which sets a 1 as easy and a 5 is very hard. A 1 would be equivalent to a land-based slow walk of 21+/- min. per mile and a water run <50 cadence. Moving up the scale to a 4, this would be a much harder land run (about 8 min/per mile) and a hard pool run (about a 75+ cadence). Get the idea? So, by switching out a couple of your land runs a week, you can perform the same prescribed workout intensity in the water (also discussed in chapter 6).
For me, my current training is still a modified venture; my joints are not happy after bike crashes (leading to a knee surgery), so land running is still a limited game. I will land run maybe twice a week, typically saving the long run for some trails, and train my tempo runs in the pool. However, when I worked with my friend Kelcie a few years ago (she was training for ironman), she couldn't run at all for a few months. I converted her land running to 100% water. When she resumed land training after healing, it was like she didn't miss a step (Kelcie's story is also in the book as well).
But, honestly, there is only one way to explore this training process; and it is for you to just give it a try. I have yet to meet anyone who has trained in the pool and not either produced the same results (but with less joint/body trauma) or was able to PR. If you try it, please let me know how it how it works for you!