There are a couple of issues here...the only truly deep cycle, reasonably
priced, flooded batteries that are readily available are 6 volt golf cart
batteries. Most so called "marine deep cycle" batteries are just truck
batteries with rope handles. The golf carts are about 62 pounds apiece so
they are generally much easier to install.

I had a house bank of 12 golf cart batteries that I took 550 amp-hrs out of
every day for a three year Eastern Caribbean cruise...they performed
flawlessly. On a $/amp-hr basis, golf cart batteries beat all comers.

Hope this helps-
Charlie