I would not attempt it in water...I would use pure perlite in poot large enough to support the plant. I bought from the same seller and my plant had no roots but the perlite did the trick. Once it was rooted I used Chanin's recipe for soil on the spiney's

HDD, how deep did you plant it?

Just deep enough to give strong support the plant until the roots formed.. then I pulled and mixed the permanent soil and repotted

I'm with HDD, moisten perlite is the best for rooting this spikey.
Give an amount of (filtered) light and heat will stimulate the root better.

I checked the depth that I thought it should be sitting in, using a 1 gallon wide mouth glass jar, filled the jar to that height with pearlite, put the plant in and wet the pearlite. It's about as ugly as you can get, but it should work. I'll put it on a piece of heat tape set at 80F/27C degrees. I'm lucky I have all that stuff here already for the reptiles.

BTW, if I'm not around much it's because it's time to bring the critters out of hibernation, and begin getting them ready to breed.

Thanks for the advice, other than a bunch of seedlings this is my first spiney.