Leaving the engine block connected to "ground" will cause a problem with current from the house battery flowing through to the block. If the block is not disconnected then as current returns to the house battery it will see two paths, one through the house battery monitor
shunt to the house battery negative terminal and another through the starting battery negative (or "ground") to the block and on to the hull and/or the sea. The purpose of adding another solenoid to the starting negative ("ground") is to isolate the starting battery charging
system from the engine block and its connection to the hull and/or the sea. There should be NO
stray ground connections to the block. With two conductor, isolated, gage sending units and some attention to the other connections, like the stop solenoid, no current should flow through the block except the starting current. Remember the question is how to wire a metal
boat to avoid electrolytic corrosion
Any good high current starting solenoid will do the job. But keep an eye out for the ground connection of the solenoid. An automotive starting solenoid may be meant to be grounded through its case mounting point to the car's frame or body and may need some isolation as well. Better yet, connect it to the starting battery positive and negative terminals and wire the starting switch (2 wire) there as well.
PS: Foolish Sailor - a gas engine won't run without a ground because the ignition current runs into the plugs, across the gap (causing the spark) and on into the block which must be connected back to the current source. A diesel
will run all day with no electrical
connections (although it may be hard to stop and won't have any gages).