Wakesetter 247 heater won't turn off
 

The cabin heater in our 2008 Wakesetter 247 is constantly on as long as the battery is connected. In order to get it to turn off we had to pull the fuse. Has anyone had this happen before, or know what might be causing it?

Also, the heater blows only cold air until the engine is being run at high RPM's (10+mph). Is this normal, or would a new heater core fix this issue?

Most likely, you have an issue with the MUX Power Module where you pull the fuse from. = Probably a need a new one $$

as for the heat itself,
That is normal, there are ways to change it to make it more efficient, however. Head over to themalibucrew.com, and search for it.

Power module is bad. Either replace it or put a 3 position switch on the dash for the fan portion of the heater.

Not sure as to the power issue as that sounds like a known Malibu issue.

But as to the heat that is 100% normal. Most boat engines do not have enough heat and flow at idle to produce good heat. You speed has nothing to do with it, just engine RPM. over around 1500 rpm you should have good heat. As some said above there are some routing things you can do if you really want heat idling all day. Most people only are idling switching riders so not an issue. A new heater core would change nothing...