I installed a jetted tub about 15 years ago. It came attached to its
own wooden supports on top of 3/4" plywood. I basically just had to
put in place and screw the plywood down. Since that time, I've been
fighting the caulking around the edge of the tub and the wall tiles.
It seems that when the tub is filled with water and someone is soaking
in it, the extra weight causes the tub to settle down. This has
caused very small hairline cracks in the caulking.
I heard someone say to get some spray insulating foam and spray it
under the tub between it and the plywood. I'm not sure if there is a
specific type of spray foam that would work best. When I went to
Lowes recently, they had two different types, small gap and wide gap.
I'm not sure if either would work here. I think the spacing between
the tub and the plywood is about 3-4 inches.
Someone else mention using a very dry mortar and then packing it in
with the tub empty and letting it cure, I'm not sure if this is a
good idea either.