You have an 18-ft open boat,right? No cabin, no 12VDC fridge, no anchor
winch, etc.? Let's not overkill things here.

I am always open for suggestions that can save me money.

First, are you SURE you need a 24VDC trolling motor? As you can see, it
complicates things greatly. Minn Kota has a nice 55-lb-thrust motor that
runs on 12V: the Max55T. I think it would push your boat fine, and will
pretty much run all day on a fully-charged group 24 battery (at
trolling speeds).

Last time when I asked around here, I came to the conclusion that I am
probably better off having a 24V trolling motor. The reasons are
something like these: (1) A 18-ft fiberglass boat with a 115HP motor is
not exactly light (around 1800-lb to 2000-lb); a heavy duty trolling
motor has enough power pulling it throgh current or wind (my boat has a
T-top that catches wind). (2) I intend to use it as a backup propelling
force in case the main motor failed to start while I am in the middle
of a lake or a bay (the main motor is 10-years old). (3) I intend to
use it as the sole propelling force when I run my boat in lakes that
have "10-hp-or-less-only" restriction. These are the reasons why I
want to choose a powerful trolling motor.

Second, if you decide on a 24V motor, you CAN buy a third
battery and put it in series with your deep-cycle battery: just
make sure it's the same type, etc. (yours is basically
brand-new,right?).

Unfortunately, the existing deep-cycle battery is not new. It is
probably newer than the boat itself. But it is not exactly new.
Therefore, I don't intend to combine the existing deep cycle battery
with a new battery to make a 24V system.

The other thing that makes me uncomfortable about your suggestion is:
If we combine the existing 12V deep-cycle battery with a new 12V
battery to make a 24V system for the trolling motor, we will only have
one battery left to do everything else (starting motor, powering fish
finder, VHF radio...etc), right? Unless I am misunderstanding
something, I will be very hesitated to do that. Please clarify.

Use quick-connect connections, and when you want to charge
them, re-arrange them to parallel. ...

I always like the concept of "Quick Connect". That's why all the water
hoses in my garden have quick-connect. How does the "quick-connect"
look like in the context of battery connection? I cannot find this
item in BoaterWorld or WestMarine web sites. Do you have a link to
this type of item?

Nice to talk with you.

Jay Chan