It would make perfect sense for one of these companies to actually come out with something that did work well with acrylic since much of the spa material is actually acrylic these days. Problem is getting something that is not absolutely permanent 'cuz if you want that - just use Weld-On 40
There is one product called ProSeal 34 that is actually made of polycarbonate and sticks like crazy to everything but is sloppy and somewhat hard to work with IME.
Still the best IMHO is Dow Corning 795 as a silicone that sticks to acrylic without being absolutely permanent.