We have a blanket so the only part of the cover that ever gets wet is about 3 inches around the edges. I wipe it down but it needs something stronger. I use Windex & a Mr. Clean Eraser Pad to clean around the top of the tub so I wil try that on the cover. I do not want to use 303(?) since it probably would drip off condensation into the spa water.
I don't understand why you think a little 303 would be a problem in your tub. I use it liberally under my cover and, when draining, will use it on the waterline and above in the tub. 303 will not cause your water chemistry any problems...as a matter of fact, I feel like it may have a positive effect as a defoamer though I haven't tried it as such. Maybe I'll give it a go and let you know.
BTW, one very nice added benefit of using it under the cover--when opening the cover, a nice, slick, freshly applied cover slides off the tub MUCH easier! 8-)