A few throughts; First it is not proper to wire a bunch of electronics directly off of the battery, except perhaps in a small skiff. You want adequate wire, connections and fusing from the battery to the console where the bus is located. Done properly there should not be any more voltage drop, and it will be far safer. The batteries are usually in an area where there is salt spray and corrosion--the smaller wires are more susceptable. The positive wires must be fused within 7" of the battery--which means more connections, fuse connection etc off the terminals and in the way of corrosion.
Most of our boats come from the factory with marginal wiring to the console--add in the older the boat the more corrosion has occured along the way. I more than doubled the wiring from the battery in the Tom Cat and C Dory 25. When you put the connection of the wiring from the battery, it should go to the battery switch or a small point near the battery which is fused. The swages have adhesive shrink wrap, and are kept bright and clean, plus sprayed with "Corrosion block" regularly to prevent corrosion. The swages at the terminal blocks or bus bars also are adhesive heat shrunk and sprayed or greased with anticorrosive gel. There are two thoughts about using switches or fuses at this point--but you do need a fuse that protects the device as well as the wiring, so the smaller manufactuer's fuse is proper. It is certainly proper to avoid any switches for the electronics, but for the Tom Cat where I run 2 radios, plus HAM, plus 2 GPS plotters, Radar, satellite radio, Audio system, etc--I put in a completely separate switch pannel for the electronics, and small bus bars for each of the electronics. No significant drop of voltage, due to adequate sized wiring, no corrosion on both plus and negative sides, and proper proteciton. With the switches I always turn all of the switches off when I leave the boat. Granted if you have only one GPS/Fishfinder and a radio--it may well be just as easy to cut the main battery switch as separate switches--and best to wire directly to a bus bar attatched to that switch.
Part of preventive maintance is going over the entire electrical system each year and checking the connections. At some point they need changing or replacing.