The chassis of the SMPS is connected to system earth anyway. So any leakage current higher than 100mA will trigger and trip the local RCD protection. If that fails then the central house 300mA RCD will trip.What I am worried about is the possibility of mains voltage somehow finding its way to the DC output of the power supply and eventually in the pool water giving people an electric shower!
If I were putting such lights in my pool I would look for a medical grade power supply. Maybe something like this: https://www.jameco.com/Jameco/Products/ProdDS/2257651.pdf
Applications Industrial automation machineryIndustrial control systemMechanical and electrical equipmentElectronic instruments, equipments or apparatus■Household appliance
I am now inclined to raplace the transformer with a higher voltage one, ie 15V instead of 12V and then use a rectifier and a DC-DC back converter.
You must choose a transformer with a split bobbin to insure the windings are separate. A layered constructed transformer could short internally and connect the mains to the output.
You need to have approved equipment for this that meets appropriate standards . Whatever was in first ...