why doesn't my cedar hot tub rot?
Cedar is often used for outdoor and other wet locations because of its high tannin content. Tannins are natural compounds that prevent bacterial and fungal decomposition (rot). 

When used in exterior applications, such as home siding, cedar is often protected with oil stains or varnishes. This is primarily to protect from sun damage, which dries out the natural oils and fades the wood. 

When used indoors, as in saunas and spas, cedar is usually left raw. Sealers tend to disintegrate quickly in very wet situations such as these, leaving it with a degraded appearance. 

In your case, I'd either leave it raw or use a penetrating oil finish that can be reapplied periodically.