Tbe tenant does not care about your property and will likely not do the right thing and allow the pipes to freeze. I am 2 for 2 on this. So even if the third tenant would of done the right thing 1 out of 3 is not good odds. Not only that but if they have never lived in a house before they are unaware of the additional cost to keep the thing heated. Keep the Hot tub empty unless you want to have a problem latter.
Now in the Colorado area I did see a hot tub advertised with a SFH and they are asking $150 more a month for the use of it. That is in addition to the rent above and beyond. For that price, if someone was willing to pay it, maybe then? There property is located in a very trendy area so they may have the right idea.
For an additional $150 a month you could have someone stop by monthly, especially in the winter and make sure they are keeping it heated.