Slinging a pool by helicopter requires a company specialising in slinging that type of large loads, either by using mirrors or preferably vertical referencing. Ensure they have the right helicopters, slinging equipment and suitably rated and qualified pilots.
Slinging it like a bath the downflow of air will double to quadruple the pools "weight" due to the helicopters downwash, to a point where the helicopter could end up in a situation where it cannot lift the "relatively light load". Turn it upside down and the downwash might cause it to "fly" in circles at the end of the strop.
Rigging, nets, strops, length of strops should also be chosen carefully and attach in such a manner as to prevent damage to the pool and the surrounding terrain and property by downwash.
Get permissions from CAA, Council and neighbours and have indemnities signed as required. Think about insurances etc.
Just a couple of pointers to think about.
It is a specialised job and damage could easily occur. If care is not taken and it can turn out to be an expensive exercise where it might leak so badly at the end that it will only be good for an extra large sand box for the kids or litterbox for the cats... hehehehe
Good luck!