New to offshore, looking for blue water info

I am new to off shore fishing as I just got a 21 foot sea hunt.

I am trying to find out where i could find out how close the trash line is and the beginning of blue water. I would like to start out fishing offshore when the run is less than 20 miles to get a feel for it and check out the boat and equipment. Does anyone know where I might get this info. Thanks, T:cheers: