First let me commend you for an excellent idea and a very creative thought for use of an old hot tub. Probably the biggest problem will be the depth you will need to dig the hole to receive the tub. The "orifices" will of course need to be plugged, except for perhaps one or two to use for draining and/or filtering the water. A paste type epoxy would work well for plugging these openings. As the hole digging is reaching completion and it is determined the tub will fit around the edges and correct depth, I suggest you hold the depth to where the tub height would be 1 to 3 inches higher than the ground grade level. This would then allow for you to choose a stone or brick product to line the edge to set the tub on. This will not only be esthetically pleasing it will help to make sure the tub, when full of water, does not settle or tilt.
Note: If you have more questions on this just get back to me.