The picture you show is not even close to be the proper amount of support needed for a tub filled with water and then 4-8 people in it. I have a J-350 and the filled weight is shown as 3,820 pounds, add just 4 people at 180 pounds each = 4,540 pounds (2 tons )
2 tons spread over an area of only 5ft by 5 ft needs a tremendous amount of reinforcement.
Add to that, a live wire with 240 Volts / 60 amps mere inches away from the water and you can see you would want to be on the safe side of over doing it.
Where I live, if a homeowner plnas to have a tub on a deck, it requires an actual architects stamp on the plans.
There are to many things to consider which are beyond a normal homeowners knowlegde, Lbs per sq.ft/inch, flex, tensile strength.
