Probably the most accurate way is to use a radioactive tracer as is used to measure blood volume but . . . . . . .
Or you use some inert chemical compound.
Look up effect on pond life, both plant and animal) of a low concentration of common salt in pond water.
A concentration of less than 5 parts per thousand ($5\, \rm ppt$) should be perfectly fine even for koi carp and indeed this is a method which is used to kill harmful parasites.
Measure the salinity of the water in your pool $S_{\rm initial}$ using a conductivity/salinity meter of which there are numerous types.
Estimate volume of pond in litres.
Add 2 kg of common salt (sodium chloride) per 1000 litres of water (total $M$ kg) which will increase the salinity of your pond by about $2\, \rm ppt$.
Less/more salt will produce less/more accurate results.
Best to use salt purchased from a koi/fish shop to ensure it is pure and without any potentially toxic additives.
Wait
Measure the new salinity of your pool $S_{\rm final}$
Volume of pool $\approx \dfrac{M*1000}{S_{\rm final} - S_{\rm initial}} $ litres.
Another way is to take known volumes of water before and after the addition of the salt, allow the water to evaporate, find the difference in weight and hence evaluate the volume of the pond.