We train statics in a kid pool, which is at most public pools here. It is much more comfortable and safer for STA training anyway than training directly in the swimming pool. Additionally, the water temperature is higher, hence you do not get cold as easily as in the big pool. Thes kid pools are empty most of the time in the evening hours, so the pool management is glad to rent it, and do not ask much money for it. Their only condition is that the safety is properly assured. In our case, we have a dedicated life guard, and additionally there is also the instructor who supervises the training and who is certified by the state.
We rent the kid pool 4 hours a week, and in comparison with the cost of the lanes we rent in the pool, it is a real bargain. The cost of the life guard is in fact higher.
When discussing with the pool managment you have to show clear documentation and plans for assuring the safety, have documented all safety procedures, have a dedicated life guard, oxygen & CPR equipment present near the pool. You have to include the description of redundant layers of safety, the description of handling of typical freediving accidents, etc. You do not forget to include regular safety drills and accident simulation (including a mocked evacuation). When you show good knowledge and preparedness, you have much higher chance to get the clearance from the manager.
In the case of our club, we have multiple rendundant levels of supervision:
1) Buddy system - we "click" each other to verify the responsivness even during easy breath-hold
2) Harder attempts are made in alternace in pairs - buddy stays on the surface and checks the one underwater regularly, pulling him out when he fails to reply the click
3) The instructor who has certification for CPR and apnea safety stays out of the water and assures the safety from outside, and takes care that everyone follows correctly the buddy system
4) The life guard sits directly near us, and has no other taks than our direct and uniterrupted supervision
5) We do regular drills of accident simulation, so that we assure we can handle a real one properly