Is having a dead starting battery so much of an issue that your really want to put a dedicated switch in place to do a bypass?
I'd attack the problem in two ways 1) focus $ and effort on the problem rather than the symptom. 2) Have a plan B. For one thing you won't have to buy as many starting batteries over time.
1) Attack the real problem. Find the source of the drain on the engine battery, using a multi-meter. Then, to help keep the starting battery charged, wire a small solar cell to support the engine battery. A small one will not need a voltage controller. It will maintain the voltage, and will even provide a small bit of charging. Think about a cell that is about 1/2 square foot in size. They sell them for this purpose, and I've seen sailboats with a solar cell built-in for this purpose.
2) For plan B, get a piece of wire that you can use as a jumper cable. Better yet, get a jumper cable. Wherever you go, you can build good karma, jumping your slip-neighbor's batteries or loaning the jumper cables to someone with a dead car battery. You may even get a beer out of it.
[Since for me, a dead engine battery happens before a trip and not usually during, there's no need to rush anything with a large amount of current all at once. I sometimes use a 5 foot long piece of normal speaker wire that I happen to have. It won't help if the engine battery is truly dead, but after waiting awhile, it get's the starter battery up to the point where it can start the diesel. The wire is long enough to not get hot. That is critical with this approach -- if it gets hot, you need a longer wire. If I need something faster, I'll rig a thicker cable for a jump. That's my method and it might not work for everyone. So it's IMHO.]
IMHO, attack the real problem and have a plan B. You'll keep the system's complexity down and have a plan B that you can apply to other uses.
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