Why does this Webasto melt fuse need changing on my boat every couple weeks. it's costing way too much, I was advised to just link it out with a piece of cable, Yes?

The Webasto DBW melt fuse is planned as a failsafe measure to prevent your heater from overheating with potentially safety critical consequences. Do not ever bypass or link out this fuse and then operate the heater unattended. If the fuse is continually blowing then please find root cause of your problem. It will always be a water coolant circulation concern, for example, failed or failing water pump, obstruction in coolant circuit, blocked water filter if fitted, or possibly air locks in coolant circuit.