Replacing my batteries for the first time on the boat. Currently the batteries on the boat are 29DC. Would I notice a difference if I replaced them with 24DC?

If you fish long hours and in the wind the answer is yes.

If you fish with 29DC now and have a certain "energy-consumption" profile for your TM, you will discharge 24DC batteries deeper into discharge to get that same level of energy from them. (Deeper discharge = bad, bad.) That's if you even have enough energy in them to begin with. If you do manage to get by on 24DC, you'll effectively be much harder on your batteries; this will typically guarantee that they will not last as long before replacement is needed once again.

Numerous battery companies have guideline data saying that batteries perform better and give longer life the less the level of discharge experienced. Batteries taken past a certain point may seem to recover, but are said to be permanently damaged and will be in need of replacement sooner. None of this is a good idea.

Thanks for you help. I'm going to stick with the 29DCs.

I just took my 4 month old Walmart 29dc TM battery back for a warranty replacement today. They gave me a new one. Was only putting out 8.5 volts and would not take a charge throwing fault code on my smart charger. This battery has always been weak and I think it was bad from the get go. Always charged it immediately after use and kept it on a battery tender.
I guess the lesson here is to get the best you can afford.

I'd never recommend downsizing batteries. Trolling or cranking.
The power output from deep cycle batteries are only stable during the first 20% of discharge (voltage). Any discharge past 80% is a detriment to the battery and definitely to any device powered by the battery. If you can "feel" the batteries getting weak as they are utilized your well past the threshold of stable power from the battery. In the case of boat batteries "bigger IS better".