There are much easier ways than adding a hot tub. They bring all sorts of problems and extra work along with them and you might find that your STR insurance payments would go up because of the hot tub. Add those to the chemicals required, the power/water usage and the labour to clean and drain them after use, then the finances are unlikely to work out.
The other point, of course, is that there’s no point spending money on enhancing your listing if no-one is going to see the listing in the first place. You can promote your listing without spending any money, just a little of your time, by using the internet (social media and so on) and networking with businesses / event organisers / other accommodation providers in your area and so on.