All the above are correct and several of the above may fix any specific fault within the system. However, it appears that your system could be dodgy given the use of a 2nd battery. You may have any or all of the following ( from Battery through to starter)
Dead Battery (first battery)
Dead Battery (2nd battery)
Oxidised Connections on Battery, Isolation switch, Solenoid/Starter (one is the relay switch attached or wired through to the Bendix Drive (cog wih spring return ) unit.
Faulty or oxidisd Actual Isolation switch within sealed unit (happened to me last year)
Faulty Relay Starter Solenoid unit.
You could have several of the above.
If this is the case it may take several consecutive Serial attempts to fix to reliable status.
If you do not understand any or all of teh above, do not worry as none of the above costs alot and any Mec/Elec or Auto Wiring person will solve relibly and quickly without specialist advise. I do suggest tho that given the possible outcomes of the machine letting you down, you get it checked out.
An automotive Elec will not be as familiar with marinising and salt corrosion as a marine speciaist. I think its fair tosay that anyone associated with the briny life will know what salt water will do to connections and different metals, electrolysis and electrolytic deposition (two different metals in salt water will act as a minute battery and fuse together, some exceptions of course)