Don't judge a battery by its size or weight, that does not necessarily mean it will be money well spent. In your application, an electric trolling motor only, in an 8' boat, what you want is either a true deep cycle (my choice in your case) or a dual-purpose battery. Marine batteries are
dual-purpose and function as both a start battery and a deep cycle.
Gel cells are not the rave, absorbed glass mat batteries (AGM) are, and they are very good. Check out this thread
for more info on trolling motor batteries.
FWIW, the Sears DieHard Platinum marine batteries, which are
AGMs, have outstanding performance and warranty (3 year free replacement) for the money. Made for Sears by Odyssey, they are one of the most desired and hottest selling batteries in the marine environment today. They are not the DieHard most of us, myself included, wouldn't buy. The Platinum is a premium battery and its performance as marine battery is top notch.
When shopping for a deep cycle battery, the number you want to use is reserve capacity
, not cranking amps. I don't know what your budget is, nor how powerful your trolling motor is, but buy a deep cycle battery with the most reserve capacity you can find that fits your budget. The group 34m Platinum with 135 minutes reserve capacity is currently on sale for $165.