I've got a 2004 Grady White 228 Seafarer that came with what sounds like the same setup as the OP's, in that it had/has dual Group 24 starting batteries. Last fall, I went out to start the boat on the mooring buoy the very last day of the fishing season and it wouldn't start. I'd purchased the boat back in October of 2021, of which one battery had a date sticker of "3" on it and the other had nothing on it at all. So, I have no idea how old the batteries actually were or how long they lasted. Anyway, I called up my local GW dealership when I noticed both batteries were in fact starting batteries and figured that as mentioned above, it would be a house or deep cycle battery and then a starting battery. My local GW dealership said to just replace everything with the same thing that was on there before, so I bought two Group 24 starting batteries from Interstate. They're 24M-XHD batteries and as the OP mentioned, I've got a battery switch that per the purchasing dealership is supposed to always be in the 12 o'clock position which I believe is "Both" or "All"... I forget which. Besides replacing the two batteries that were of an unknown age, I haven't had any issues with the boat at all, especially in regards to the electrical/starting system. For those that are suggesting going up in size to a Group 27 battery, one consideration and/or potential issue could be the size of the larger battery in that at least for the Interstate brand... a Group 24 24M-XHD is 11" long whereas the Group 27 27M-XHD is 12 3/4" long. I believe, though it was almost a year ago when I looked into it and don't remember for sure, that the Group 27 batteries were going to be too long to both fit in the same battery compartment where they're currently located. As well as they'd have to be separated one per each side if I did decide to go with them. I was replacing the batteries out on my mooring buoy and went out there with my kayak, so I didn't want to change anything up like that at the time, and honestly I don't see the need to increase Group sizes on batteries, at least for my particular setup. The only difference I can see with the Interstate brand of batteries is that they have a longer reserve capacity (RC), but the cold cranking amps (CCA) as well as the cranking amps (CA) are both still the same at 800 CCA/1,000 CA. But hey, it's your boat so do what you want to do, though I'd maybe make a call to your local GW dealership to see if it's even needed in the first place before starting a project like that.