If the brass fitting is defective you should look for "Whale Wx1533B - Adaptor Brass - 1/2" Female To 15mm". Usually the brass element is OK but the plastic insert which "locks" the hose in the fitting is defective or the rubber ring gaskets. You may disassemble the brass fitting. You only need a small parallel screwdriver. There should be a rubber ring gasket at each side of the fitting and one grey plastic insert. At our case the grey plastic insert which "locks" the hose was defective. The white plastic ring at the end of the adaptor does not appear to have any particular function.
If you have another Whale fitting you may take the rubber ring gaskets and the grey plastic insert (lock) from this adaptor and replace the worn parts of the brass fitting. The rubber ring gaskets and the grey plastic insert should be the same throughout the whole Whale system. I have done this on our boat as I no spare brass adaptor was available. It works well on our boat for more than two years now.
There is video on youtube (
https://www.youtube.com/watch?v=0-srEmBIshM ) that shows how the adaptor is made. The video may help you to understand how you may disassemble the brass fitting.
Note, Whale says that the maximum temperature for their system is 65 degrees Celsius which in our case was the problem as the thermostat of the calorifier was defective and did not switch of the heater. The water was boiling all the time. You should make sure that this is not the problem at your boat as otherwise the problem may occur again shortly.
If you do not have a spare Whale fitting I would replace one from the cold water system with your provisional measure. Otherwise I would be concerned that the hoselock you use as provisional measure will not stand the temperature.
I attach a pdf which appears to show on page 2 all parts of Whale's quick connect systems. Maybe it helps. Good luck.
Best regards
Holger