I found this site - there's a list of Polish manufacturers of swimming pools:
I believe you can build an own pool on your private area while maintaining the basic safety standards.
According to lakes, nobody will harpoon you. Just look if there's no swimming prohibition. I never saw any abandoned net in water where people were swimming. So look for places where people are present and you'll be safe.
An interesting article here about the VAT cost when building an own pool:
In short, there are two possibly VAT rates for such constructions: 8% and 23%. And which one will be applied depends on how big is the flat or a house by which the pool will be constructed. And also the second question if the pool is permanently connected with the home or may be transferred to another place. In this second case the VAT is 23%.
Regarding the size: if the building or an apartment, where (or in which) a swimming pool (or minipool) is built, does not exceed 150 sq. m for apartments and 300 sq. m for houses then the tax rate is 8%, otherwise the 23% VAT will be applied.
And one more thing. It is advised to purchase the full building service:Those who build houses less than 300 sq. M, and want to have a pool, should order the entire service in one company, including the purchase of equipment and then their assembly - then they save on tax.
If they make a mistake and buy eg. Jacuzzi bath separately, and later arrange the installation, then they will pay the lower VAT only on the assembly service.