What he said, sonotubes are for setting footings to below the frost line. That varies with where you live. Water weighs about 8lbs/gal, plus the weight of bodies and the tub. Overkill design is never a bad thing.
Hot tubs are often set at or near the deck level so you just step down into it. If you want it on the top of the deck, you need to at least, double the deck support structure(double 2x joists, extra blocking, and footers.
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