Both batteries should be grounded to the engine block since that is the largest metal mass on the boat. Perhaps the transmission is the same or perhaps not. I'm not in a position to form an absolute conclusion.
B+ means the 'positive' voltage or just plain 'positive/+'.
The first two statements you quoted are saying the exact same thing. In case you are confused, the first statement is about the intended objective and about not about describing the problem.
All grounds are common. All positives will be common per the above instructions. However, there can be resistance inequities based on distance and gauge. And, resistance equates to a voltage differential. Not knowing where your batteries are located in respect to the audio equipment and namely the amplifiers, the safe route is to get the source electronics + & - from as close to the amplifier primary + & - terminals as possible. Of course the amplifiers will get their + from the dual battery switch and their - from the battery.
So you can ignore just about everything and just follow the simple instructions listed above in my prior post.