Chris, I get my ferrite clamp-chokes from a local electronics
surplus store (but I'm an electronics
geek). I see that Radio Shack has something that looks reasonable:
Snap-Together Ferrite Choke Core - RadioShack.com
These are generally available in larger and smaller sizes. There are different ferrite core
materials available, some optimized for VHF interference
and some better for HF (SSB) frequencies. It can be hard to get this level of detail for the cores you might find, but any of them should help (if the problem is interference radiating from the cables).
I would buy a handful of the chokes and put them on any cables
as close to the device as you can. Use ty-wraps to keep the cores from sliding, and you may want to clamp the cables down to something solid, to keep the heavy cores from stressing the cable connections as the boat rocks and rolls.
If you can make several loops through the core
, be sure to pass the cable through the center hole, around the outside of the core, then back through the hole (wire always running in the same direction through the hole). If you run the wire back through the core in the opposite direction it will cancel the choking effect. Of course with #8 wires, you will probably only be able to fit it once through the core.