On my boat (B36/2002) the calorifier is plugged into a mains socket under one of bunks. First thing before doing anything else, is to pull the plug so that the electricity to the heater element is absolutely cut off, no if's, but's or maybe's about it. Then to be doubly sure, unplug your boat from your shore power supply.
Next, you may be able to get a shallow tray, or hopefully a deeper tray, under the end where the water connections are located. Prop up the forward end of the tray as needed to keep the tray more or less level.
Make sure that your domestic water pump is turned off, and ideally if fitted close your water tank isolating valves to prevent or at least reduce any subsequent leakage or seepage where a system might not be as tight as hoped.
One at a time slacken off the domestic fresh water inlet and outlet connections to the calorifier, allowing the water to trickle into the tray, and with a sponge, bale out the tray into a bucket before the tray overflows into your bilge areas. When no more water comes out of the first connection that was slackened off, leaving it slack, then start to slacken the second connection which may allow more water to trickle out of the first.
Check that there will be enough space in front of the calorifier to withdraw the electric heating element, if not, you may have to unbolt and move the calorifier sufficiently to allow for heater removal, and that might mean having to disconnect the hoses from the engine cooling water system. When no more water comes out from the slackened connections, then start to unscrew the heater element. With the faulty element out, replace it with a new element and re-secure the water connections.
Make sure that the work area is properly dried out, and then open any isolating valves and start your water pump. If there are no leaks, turn on one of the taps which has been set to provide hot water, and clear the heater of air. Attend to any water leaks that might now appear, and when you are sure that the pipework is free from leaks, then plug the heater back in to the socket under the bunk. Plug in your shore power, and switch on the heater for the calorifier.
Hopefully you now have success, and your bilges have remained dry.