Hey I actually started doing this, it's the most reliable way to slide with D-smash. The pivot input is really tricky though:
(shellshift)
(to pivot), or you can let the control stick snap back to neutral position which is less reliable
(to d-smash)
It's also an amazing way to slide in with shield since there is no lag (unlike wavedashing in). So hydro-pivot (what we should all call it, not shellshine) is actually really cool, but probably as hard to do consistently as hydroplane-forward-up-smash without tap jump on.
This definitely needs to be in the AT list, it's really cool, and you can control the distance you slide by how much you cross the horizontal neutral over to the direction you face post-pivot.
Another use is to hydro-pivot into bubble if you can b-reversal it, which is an absurdly crazy input sequence.
shellshift
pivot
bubble
b-reversal
...We need a new AT thread.
All of this only further shows that squirtle's true neutral game is in a shellshift, and for optimum potential you need to constantly moving around at max speed (wavedash, waveland, shell-sling into waveland,), coming in and out of shellshift. Damn this character has a higher skill ceiling than even I previously thought!