OK - sorry - here's some info:
Yes, you will need to have the sub panel in the system for a HotSpring, Tiger River and Caldera spa. You can't re-wire them for a direct feed from a single 50A 2Pole breaker.
Go to
http://www.hotspring.com/cgi-bin/download.cgi?c=4&f=pdfs/pre-delivery_instruct_04.pdfClick hereThere are wiring diagrams available for various years on the HotSpring web site under 'Customer Corner.' You can also download owners manuals etc.
This does scare off some potential customers who are replacing a spa which did not have such wiring - but it really shouldn't. I think the way Watkins has chosen to supply power to their spas is about the safest way to do it. Also, it has been my experience that you will have to do some changing to the wiring anyway - either the wire comes in the wrong side of the spa, or you may want to move it a couple of feet because trees/shrubs have grown or been removed, or that neighbor has a yard light now...
Getting back to the question at hand, I usually offer to do the wire change-out (I'm licensed) in the case of a customer who is concerned about extra cost. What I do is to simply divert the power which would have gone directly into the spa to the sub panel, and then I set the sub panel in a creative location - such as under a step, behind a decorative panel, around a corner, or some other way to get five feet distance from the water to the box. Sometimes just putting it a foot away and covering it with a planter will satisfy the inspector. I have found that our various city inspectors would rather see this level of GFCI protection, and they appreciate the fact that in a retrofit application it may be harder to get the required clearances. They will accept a "reach radius" which means that if the box is too close but you have to reach around something to get to it, they will measure the distance you have to reach.