Hot tubs are heavy. You cannot put them anywhere you want. You certainly shouldn't sing, "He ain't heavy, he's my hot tub..."
Clients often ask me how heavy a hot tub is.
A four person hot tub holds 300 gallons, or so.
A six person hot tub holds 500 gallons, or so.
And an eight person hot tub holds 700-800 gallons of water, or so!
A gallon of water weighs 8.35 pounds, if I remember my 9th grade lab science class.
So, looking at this picture, this is a six person hot tub and holds 500 gallons.
That's 4175 pounds of JUST WATER!
This hot tub has a wood exterior, fiberglass interior. It weighs almost 1000 pounds.
If you fill it with six people, averaging 175 pounds (conservative estimate), that's an additional 1050.
Theoretically, full of people and water, this little fellow weighs well over 6000 pounds.
A 6000-pound anything needs a really sound, strong foundation.
Your basic, average, even professionally-built deck, is not made to hold 6000 pounds in an area of about 50 square feet. And certainly it cannot hold that weight continually for a long period of time. 
This deck was founded very well. It is supported by double, built-up beams, resting properly on 6x6" pillars, with 2x12" joists, spaced 16" apart with blocking between each joist to prevent racking!
THIS IS A WELL-BUILT DECK! BUT DO YOU SEE THE SAGGING BEAM?
I took the picture above by resting the camera on a level step away from the hot tub. It might be a bit hard to see, but that deck is leaning downward, to the left.
Can you see how far it has sunk below the bay window on the left? About 4"! And that sinking is happening just around where the tub is located.
THE TUB IS SINKING. IT HAS BEEN THERE FOR 3 YEARS.
Another couple of years might have been catastrophic!
So why is it happening?
The column in the immediate vicinity of the hot tub, and beside that bay window, is obviously sinking.
An engineer friend of my says that once something starts sinking, it will usually continue.
Given the professional nature of the deck, I am going to assume it was installed properly.
That means it is 24" deep, with a 12" concrete footer onto which a base is attached and the column is secured.
EVEN IF THIS SINKING COLUMN IS FOUNDED PROPERLY, IT IS NOT INTENDED TO HOLD 6000+ POUNDS INDEFINITELY.
And so it is sinking.
And it's too bad, because this is a wonderful deck and the repair will require a complete rebuild of that beam and column, even after the hot tub is removed.
My recommendation: before you do something like add a hot tub to a deck, get some professional help to determine what it would need to make it safe and secure in the long run. After all, gravity works! Every time.
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