You sure can use a regular car battery to run it.......for now.
It's kind of like using a pipe wrench to drive a nail; it'll work but won't get the job done quite as well as a hammer and you're going to mess up the wrench in the process.
The only thing you have to be aware of is the limitations compared to a deep cycle.
Regular batteries are "starting batteries". They are meant to give lots of amps over a short period of time. IE: starting an engine.
Deep cycle batteries are intended to give amps over a longer period. IE: tooling around the lake all morning.
The other area of concern is recovery and battery life.
A deep cycle is intended to be run down very low on charge and recharged many times. A starting battery will run down just as low, but you will take life off the battery each time you do it and recharge.
The good thing about your current setup is you also have a gas motor. If the battery dies, you still get back to the ramp. So running the current battery is a viable option. It just may not last as long on a day's use as a deep cycle.
To sum up, it'll get you by but it won't give you the same performance as a deep cycle and will require replacement sooner.
I'm in the middle of a build myself but will be running a deep cycle with a small solar charger and plugging in a battery minder while in the garage. I will also only be running the lights, stereo and pumps off it right now. No trolling motor yet....