Marine electronics are hard to find over there. So far i have struggled to find any other than real base level stuff in tackle shops. There doesnt seem to be nearly as many recreational boat owners there as we have here - most Japanese people would struggle to have room to store them unless they live on a farm or are very wealthy and have large houses and area for storing boats off the street.
I've found some excellent tackle shops in Tokyo and Nagoya, but you need to know your prices - things like reels etc generally not much cheaper, but the selection of rods and lures, especially things like squid jigs, is mind boggling, and well priced. They aslo have a second hand section in many shops where gear that has been tradd in can be bought, some very tempting gear in great condition can be found.
Which areas will you be visiting?