Solid wire connections are crucial for safety and performance.
Simply put, while twisted and taped stranded wire will work, it puts a lot of resistance in the circuit. As time goes on the wires will corrode, which then further increases the resistance. This resistance is converted into heat and will eventually burn whatever is around it. Same concept as using wire that is too small for the application. In that case the entire length of wire is the resistor and will get hot.
This isn't as critical for lower-draw electronics like the fish finder, but it is very important for the trolling motor which draws a lot of current. You'd probably want a minimum 10-12 gauge wire for a 35-40lb trolling motor. Splices/connections should be properly soldered or crimped and insulated/sealed.
Also, a fuse or breaker should be installed close to the battery and another one at the device. This way if a short happens somewhere between the battery and the device, your terminal connections won't catch on fire again