plug for 1996 35 fountain lightning
 

I found a rectangle plug under the rear seat above the speaker cover with two male prongs, I assume this would be for jumping or charging batteries without opening the engine hatch?

Yes. Same setup some tow trucks use. Under the seat you should see leads going from that to a battery switch. My other end has the femal plug with a set of jumper cables

Any marine store have them?

Not a marine item, jumper cable disconnect. Probably find it on Amazon if not aw direct

Thank you

Found on fork lifts.

This.