www.iFourWinns.com

I’m considering adding a house battery. Right now I have one battery for port engine, one for starboard engine and one for house. Has anyone added another house battery to their 318? Which manufacturer/model is recommended? How hard is it to wire in an extra battery?

It is not hard at all.
I personally use Sams Club for all my marine battery purchases.
Walmart Deep Cell Batteries are good too.
Just make sure you match the size of the existing one (and if it is more than a couple years old, you might want to replace it as well). You need to keep the two batteries having the same capacity.
I used Size 31 Deep cell.
Make sure you have room next to the existing House Battery.
Make sure your battery charger can handle the extra load.

To Wire.
Purchase two (Red and Black) (2- Gauge) battery cables at Walmart that are the shortest length that you will need. Hopefully you can get 2 that are about 12" or 18" long and put the batteries right next to each other.

When I did mine - I also had to replace the battery charger (I had a 278 single engine). You might just consider purchasing a separate 2 bank 20 amp battery charger instead of using your original one that chargers your engine batteries.

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The Dance II
2007 278 Vista
6.2L Mercruiser

They have a bunch of different configurations so you should be able to find something that will fit. As TheDancerII said you need to have 2 of the exact same battery and also both should be new otherwise the older battery will be parasitic on the new battery having a lower capacity from age.

I installed Northstar AGM's last year. They are amazing. They are about double the price but they last 3 times as long as lead acid. The estimates are 10-12 years so if you plan on keeping the boat AGM is the way to go. Just verify your charger is compatible with AGM chemistry. I went from one battery to two 180 amp/hr batteries and they are almost the same size as one 8D. They have standard sizes but also have slim designs like a 4D and smaller.
No affiliation to Northstar other than a good experience this year. There are many other good companies but a bunch of bad ones so do your homework.

One thing to watch is your the weight. Adding a bunch of weight to one side could give you a list or worsen one that already exists. I added all this weight because I was trying to remove a list which worked out for me.

I also installed a cheap digital voltage gauge in the cabin to monitor the battery. It worked great. There are voltage to life charts online to help from going below 50% capacity.

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--2002 3880 Regal Flybridge | Twin Merc 8.1HO **Seven**
--2003 AB 13 VST Console Inflatable | Tohatsu 40hp TLDI **Phish'n ski**
--310 Mercury Inflatable | 1982 9.9 Johnson