You can find a 24v battery, but you would not want to pay for it. Get 2 12V
deep cycle batteries. Walmart sells them for about $50-60. Warranteed for
2 years full replacement I think.

This explains the reason why I cannot find any 24V battery in local
marine supply stores. This means I should forget about using one
single 24V battery, and should concentrate on getting two 12V batteries
that are much less costly.

Run the trolling motor off these batteries, and keep a set of jumper
cables in the boat and if the single starting battery is dead, jump it.

I assume the way to jump a dead battery in the boat is exactly the same
as jumping a dead battery in a car (honestly I had done this multiple
times to my car because I tend to forget to turn off the headlight).
This sounds easy enough. Thanks for this suggestion. This means I
will have a set of backup batteries ready to go in case the starting
battery goes dead.

If you already have duel batteries, just run the trolling motor batteries separate.

This sounds like a good way to avoid messing up the existing battery
connection while still keeping the option of jumping started the dead
starting battery with a set of jumper cables. This is quite likely the
way that I will go. Thanks.

Jay Chan