You might want to take a look at these...
http://www.airmar.com/pdfs/cat/marine/inhull/M260.pdf
The new M260 transducers (inhull and thruhull) are a real step up over anything out there. I was out in a boat last weekend with one of them in it, and we where marking bottom in 200+ at 35 mph. One of the best setups I have ever seen.
I will be putting one in soon.