My fiberglass boat had a 'puck' style transducer epoxied to the inside of the hull when I got it. When I restored my boat, I reinstalled a new one in the same spot, but I was never that thrilled with its accuracy.
I replaced the puck with an external transom-mounted transducer and I like it much better. In addition to more accurate soundings, it also reports the water temp, which is nice. (I believe I have a Helix also; I replaced the old Humminbird with a new one and I keep forgetting which model I bought!)
If you only want to know the depth, you probably can get away with a puck transducer. If you want to locate fish and underwater obstacles, though, I think you will want an external transducer.