For a great (IMO) combo GPS and Fishfinder unit at a really good price (around $200 USD), you might want to consider the Eagle Cuda 350 S/Map. Compact design, fits onto a small boat nicely, and is "good enough" for the needs you are likely to have on your boat.
Of course if money is no problem, there are an awful lot of other choices that are far superior, but in terms of bang-for-the-buck, I don't think you'll find a better deal. I've been running the 350 (and its predecessors) on my kayaks and my 11' Avon RIB for years and they are very easy to set up and use, reliable, and amazingly full-featured for the price.
I also use the Navionics app for the iPhone as a backup when I am along the coast, since the chart data is much more complete than what you'll find on the S/Map. But I'm usually within 3 miles of the coastline and just hitting fairly shallow spots (less than 50 meters) for fishing and free-diving/spearfishing. The 350 S/Map fits the bill perfectly for those purposes and makes it really easy to save spots and navigate back to them.