I think the notion of "needing" a deep cycle battery for a trolling motor is a bit of a misnomer. The deep cycle aspect is more intended for cold cranking power. More amps available to start an engine. A new 60 month battery should work fine for simply powering a trolling motor.
Everyone has their theories, so here's mine!
My understanding is that deep cycle batteries are intended for uses where the battery is drained down significantly before being recharged. AND they are intended to be re-charged over and over. A regular car battery will not take that kind of continuous abuse. They are made to have engine starting power and then be maintained in a fully charged condition continuously.
Here's the other things I've read about maintaining your deep cycle battery:
Re-charge immediately after use- don't leave dis-charged for long periods of time.
Check water level and add if needed AFTER charging. (don't overfill- to the "slot" only)
Keep in cool (but not freezing)location- a cold concrete floor does NOT drain a battery (old wives tale)- it's actually an excellent storage place.
Better to not drain more than 50% before re-charging, battery will last longer. ( I violate this on a continual basis- everytime I use it!:naughty
That is my .02
Oh- and I did used to buy cheap car batteries for the electric trolling motor- they didn't last long and I wouldn't waste the money to do it again.
Jim