Please note that while the term "marine battery" is oftened used as a
synonym for "deep cycle battery", manufacturers also make marine
starting batteries which are not deep cycle. Starting batteries (such
as jump start batteries) have high cold cranking amp ratings, but do
not tolerate deep discharge without damage. A marine starting battery
is just a starting battery built to be resistant to the pounding
sometimes experienced in boats, and is no better than any other
starting battery for deep cycle use.

What you want is a deep cycle battery, marine or otherwise. Wheelchair
batteries (available at Sears, etc.) are deep cycle batteries and come
in various sizes which would probably meet your portability
requirements.