Re: jump box for back-up starting
A jump box will not always work if hooked to a defective or low battery. The jump box sents power to the battery instead of the starter. Sometimes you have to disconnect the main battery before using the jump box.
If you are going to use the jump box for other things besides the boat they are nice to have. If you are going to only use it on the boat, a second battery would be better.
Mike