It is unusual to have sensors in the pool in a way that is accessible to users, partly due to the risk of damage or misreading during waves/splashing, normally the level is measured else where in the plumbing. Then in the same way that mains pumps are OK, because there is separation that limits the current that could escape into the pool itself via a length of (often earthed) pipe  or whatever, the situation is a lot safer.

However, the voltages and currents quoted are not in themselves dangerous, the question is how credible failure modes of the sensor or rather the power supply might play out.  An RCD on the mains side will do no harm, I am surprised there is not one already, and  I assume the mains does not go into the pool, rather there is some sort of power supply associated with the controller, safely in the dry, and the sensor is on its own lead from that.

It might be sensible to check with the makers of the box that converts the mains to the 5V or whatever, that this is an application they are happy with. In the meantime your approach is correct to be cautious. Just having 'worked for years', is no guarantee it is unconditionally safe.

Mike

Thanks Mike

Yes I suspect these level sensors have been fitted by in house techs and are a bit of an after thought as another pool onsite that is newer has sensors fitted to the balance tank via a site/level tube utilising a floating metal ball and type of magnetic switch clamp to the outside of the tube.

Thanks for the pointers, it is appreciated.