I'll echo a lot of the statements given here. My own experience with Interstates is they aren't very durable. On average, my car and motorcycle batteries have lasted a minimum of 3 years with most going well beyond 5. The Interstate Deep Cycle battery I have lasted 25 months (a bit over two years). To me, that's lousy. Well cared for lead-acid batteries should last at least three years and, on average, five (my own personal belief). When I went to look for a replacement Interstate, I was looking at a $95.00 price tag (ouch!). Ended up buying a new Bass Pro Deep Cycle for $69.99.
Go ahead and get a new cranking battery. Bass Pro has them for Marine use as does WalMart and Advance Auto Parts. I believe Sears even carries some Marine batteries. Dick's Sporting Goods sells batteries under the Minnkota brand - but they note on the label Made by Trojan so you get the same battery as the highly rated Trojan, but at a slightly cheaper cost. Make sure to buy the right battery for the right application - marine cranking battery for engine and accessories, deep-cycle for trolling motor. Good luck!
JC