I think you may be needing a battery for a trolling motor based on your post, in that case you need a deep cycle marine/rv battery. If strictly for starting the motor you need a marine starting battery or you could use a marine deep cycle/cranking battery for starting you motor and using a trolling motor off the same battery. There is definitely a difference in battery design for these different applications, e.g. bigger lead plates, more lead plates, etc.
I have purchased the last several batteries for my boat from Advanced Auto. Don't know if you have one in your area, but their batteries have a 1 year warranty and you can usually get a 15 to 25% discount if you order on line. If your local auto store has it in stock, you can pick it up the same day. Advanced auto has about the same prices you quoted before the discount. Hope this helps.