If the fuse is blowing, then this isn't a voltage issue, it's an amperage issue. While the devices you're using are all 12V, they are pulling more amps than the fuse you're using can support. A 12 volt device that pulls 10amps is going to blow 5 amp fuses all day long.
The device is going to draw the amperage it needs. The fuse is set to burn out when the amperage draw exceeds the melt point of the filament inside the fuse. The idea is to protect the device from a surge in amperage and sacrifice the fuse before doing damage to the device.
I would verify the amperage requirements of the device(s) you intend on using with the outlet and then make sure the fuse in that slot will support the amperage requirements of the device. Admittedly I'm not an electrician and barely understand enough to be very dangerous. As with all forum replies, take them with a grain of salt and consider seeking out a qualified marine electrician who understand 12v marine electrical systems if you're still in doubt.
I recommend reviewing the ABYC site for a list of local ABYC Certified Marine electricians:
http://www.abycinc.org/
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