Golf cart batteries
I have 6 Trojan T-105''s in my boat. I have them configured in series-parallel as described by boatbum above, then in banks of 4 and 2. I use the 2 battery bank as the starting battery and the 4 battery bank as the house battery. The bank of 4 gives 440 amp hours of capacity, so if you go this route, a 100 amp alternator is about right. I have a Balmar 100 amp with their 3-stage regulator. Seems to work pretty well with my battery configuration.
I would recommend these batteries as good value for the money. I paid about $75 each about 5 years ago. I have had to replace the batteries about every 8-9 years with usage mainly on weekend cruises with a fridge being my biggest draw. I have tried "marine" starting batteries and gotten 2-4 year service so in my experience the golf cart batteries have been a good investment.
I''ll definitely use the T-105''s again (hopefully not for several more years) although I will probably reconfigure to the more recent recommended way of doing things and do away with the 1-2-Both switch in favor of a totally automatic system like discussed here: http://www.amplepower.com/primer/prefer/index.html
There''s lots of info available on this subject on the web, just do some Google searches.
A disadvantage of the 6V batteries is extra connections to wire them in series-parallel.