We don't think much about batteries until they fail to perform. Most of the time this situation is self inflicted.
This is a matter of amp hours. How much do you think those DRs run? If you know the draw in amps multiply that by the time they are running in hours. Most Deep cycle batteries are 60 - 100 amp hours. There are bigger ones that go as high as 200 amp hours.
If you do that math you will see that you are probably less than ten minutes of runtime per hour. At 30 amps (way overestimated, worst case) you are consuming 5 amp hours to run each DR for every hour of operation. About 8% of the lesser deep cycle battery. You would make the same estimation for electric reels. They run on a higher duty cycle so the amount of power consumed is higher.
Typical outboard alternator is between 30 and 60 amps. So you could replace 50 to 100% of the battery charge every hour. Even a kicker will put out enough, between 6 - 10 amps to outcharge the draw from one DR. When you get two or three then you have to sharpen your pencil and maybe worry a little bit.
The bigger issue here is your battery switch. It is always a good idea to have a battery that is not discharged when you most need it to start your motor. Typically I isolate the starting battery from the system by selecting the other one on the battery switch while I am loading the system with electric reels, DRs or pot haulers. I can discharge the 'house' battery as much as I want. When I get ready to run I select the other battery and go. If you have a problem that discharges your batteries and you do not have a fresh starting battery to bring on then you are not starting your engine.
Last thing this time about charging. A discharged battery will put a serious load on your alternator. If you really run one down you might want to switch it off until you get home and use the shorepower to charge it. That would put less stress on your alternator.
There is always the ACR if you want to not manage your battery charging. And newer outboards have two alternator outputs that can be set up one for each battery. Your boat manufacturer figured most of this out for you. Or maybe not.
Here's a question. To add more downrigger receptacles to use for electric reels it should be ok to daisy chain if that's the proper term from the original two instead of running new wires back down the the batteries, blocks et cetera. Correct??? The extra receptacles would only be used for electric reels.
In a word no. Use a separate wire run to the outlet and back from it. At 30 amps you will need 10 ga or maybe larger wire depending on wire run length. If you use another tap on your first wire run to power another outlet you have at times doubled the draw on the circuit. More voltage drop, less power for the reels. Imagine "everyone reel up" before you go to the top of that next drift. When you kick that second reel into action the first one will slow down and your wire will possibly heat up.
