Gee, the concept of carrying a backup battery, only good for jumping the main battery, makes no sense to me. Batteries last a certain amount of time, say 5 years in a boat. If you buy a new battery and a jump packet at the same time, they will likely both go bad at the same time.
If you wait to buy a jump packet, while your battery goes bad, you are better off buying a new battery.
I suppose buying a jump packet, when your batteries are half-way work out, has some merit, but the economics, space in the boat and weight don't work for me.
Also, batteries will normally not work on cold start, as that takes the most current. That should be at your dock, or on your trailer. Those are the best places for the boat to be, when the battery fails, as it can be replaced most easily.
JMO....