I have a Coleman Scanoe. I have a Humminbird PiranhaMAX 160PT that came with the external transducer. I used the fish finder a total of three times and something nicked the external tranducer cable to the point of being beyond repair.
From the start, I had a hard time keeping the transducer attached with the suction cup that I was told would hold on to anything and would be difficult to remove. Wrong. It wouldn't stay attached to my Scanoe for more than 15 minutes (polyethylene hull). I had to keep reattaching it time and time again. I later talked to someone who had the same problem and he said he made an attachment to his boat or canoe (can't remember which) out of plexiglas as that is the only thing he could get the suction cup to stay attached to.
Once the cable was nicked, I replaced it with the Humminbird XP 9 20 T Puck Transducer. This mounts inside the hull of the canoe. It comes with a slow cure epoxy to permanently attach it to the hull. However, since I am not sure if this will be my only boat, I use a putty that I found laying around the house to mount it temporarily and it works quite well. You just have to keep the area around the puck clear so that nothing knocks the putty and puck out of place.
The Puck Transducer has a temperature probe, that can be mounted to the transom with small "clamps" that are included so it extends into the water. I am not always concerned about water temps - and if I am, I manually lower it into the water. To me, its just another cable that can get nicked by whatever did in the last transducer.
According to what I have read, reading through the hull is not as accurate as if the transducer were in the water. It seems to work o.k. on a lake that I frequent. However, when I was on the Potomac River this past weekend, I had mixed results. At one location, which was fairly weed/grass free, I got excellent results (the water was clear enough for me to compare finder readings to what I could see). On another location of the Potomac, there was so much weed/grass that I was not getting accurate readings. I am not sure if it would have done any better if it had been an in-water transducer. The weeds/grass were very thick and probably caused the inaccurate readings.
I hope this helps a little.
Laura (aka Zeebyrd)