Do your test borings (probably 2 will be sufficient) in the shallow end and the deep end. The depth of the holes should be about 5 feet below the deepest end of the pool. For example, assume your pool in deep end will be 8 feet, then your boring depth should be about 15 feet below existing ground level. Before my geotech buds chime in that I'm all wet about the depth, here's my thinking.....you will be removing soil when you dig for the pool, then replacing with a relatively thin layer of concrete and 6 or 7 feet of water. The net loading on the soil below the pool will likely be nil or even less than before installing the pool. The primary concern will be where the water table is at the time of construction and where it can fluctuate to during the year. It is fairly rare for a water table to fluctuate more than 5 feet or so during the year (yes, it happens, but not everywhere and not every year), so that's my reasoning behind the 5 feet + pool depth for the minimum boring depth.

If the water table is within the pool depth, you'll have to carefully dewater for construction (don't cause your house to settle from the dewatering for construction!) and keep the groundwater below the pool depth by a couple of feet during the construction and the time it takes for the concrete to gain strength. When the pool interior surfacing is done, you can add water to the pool and allow the groundwater to come back up at similar rates and you shouldn't have any significant differential stress issues.

Ron maybe right, but I have another take. There may be the time you need to drain the pool and of course you don't want the thing to float. You may find that soil conditions in the area are such that gravity drainage from the pool's drain will work fine. Finding that location may require some work. However, if soil is tight and it doesn't drain, then you should be prepared to actually draw down that exterior water table at ay time you wish. Therefore part of the construction should be some form of under-pool granular drain system, say to a man-hole where a mechanical pump can lower that external water table while the pool is empty. Also some pools are designed so the pool can drain via a one-way valve, letting ground water in so it won't float. Do not depend on that. A stuck trap valve has been the cause of a few large pools I know of to float.

Thanks so much Ron and oldest guy!
The pool is rectangle 6m x 3m so will take a sample from each end. greatly appreciated