Just a few hundred mA is almost always fatal in a pool the voltage does not matter, if its passed directly through the heart 200mA @ 9v could probably kill someone.
Any isolation transformer based lighting system adds some protection at any voltage since you need a second fault or wiring mistake to become lethal, I am not sure if 12V systems have some type of GF protection on the secondary side.
The only thing you can do to check a old pool for safety is to test the equipotential grounding grid, equipment grounding conductors every few years and test GFCI(s) per manufacturers instructions.
The biggest safety enhancement you can do for a older pool is add a 30mA GFPE breaker on the feeder to the pool panel, so if any GFCI fails 'on' (or gets replaced with a regular breaker) you have some fallback.