The water heater that came with my home when I purchased it is on a double pole 30A breaker and wired with 12/2 romex. The breaker seems to be sized correctly for the wattage (4500W) but should it be using 10/2 wire?
Make sure you wait for someone with CEC knowledge. It seems some provinces include water heaters as heating equipment and apply CEC 62-114(6) and (7), which I think eliminates the 125% continuous load requirement for the wire if the wire is at least 80% of breaker. So 18.75A in a residential application would allow 20A wire, maximum breaker of 25A.
In the US the Installation Instructions are part of NRTL Listing, which override the NEC. The Installation Instructions from WHI (Rheem) say 25A breaker and #10, so we would have to use #10.