Every one has their own opinions as to whether it is worth it etc. I have had mine for less than a month and it has really helped find mac bait balls deep in the water column. As for marking actual fish, don't know if mine works for that. Some say buy the cheapest one becuase it isn't going to last anyways, while others like the expensive ones to get a better picture.
My transducer is just marine gooped to the hull. There shouldn't be any bubbles under it. I run it off a 12 volt 5amp hour SLA battery that I keep below. My wiring isn't fancy, I just run the wires into the front hatch, and close the hatch over them. I may lose some seal but it works well enough for launching throught the LJ surf. The mount is screwed into the deck, with marine sealant to make it water tight.
I am not an expert at this. Some others will have some more informed opinions.