Are you using for fresh or saltwater?
Do you want to see actual fish or just find the bottom?
If you go in the salt you need at least a good quality (i.e. Lowrance, Furuno, Simrad, Raymarine etc.) dual frequency 50/200mhz sounder with at least 600x800 pixel resolution or higher to have a chance at seeing real fish (i.e. fish arches), not just stupid fish symbols which could be anything or nothing. In this case be prepared to pay between $1,500 to $3,500. If you just want to see the bottom then just about any $500-$1000 unit will do. But if you just want to see bottom why bother with a colour unit. That my 2 bits. Good luck.