Any of the Humminbird Matrix or 500 series are good. And, you don't have to spend a bunch of money to get a good one with lots of resolution. Lowrance makes some great units. I don't have one on my boat right now but will have in the near future.

After you get it, find the post or read the article on my website Jerry Blake wrote about getting the most out of your sonar unit. It will help bunches.