I know some of you have such devices mounted on your kayaks. Any recommendations on a decent quality unit that'll accept a Navionics map chip, and doesn't require too much hole-drilling on the boat?

Depends on if your kayak has Geartrac or not. If it does then no holes at all. If you don't then most require at least 4 holes for the mount. If you don't have the gear rails then I suggest getting some and mounting those. Then you can put all kinds of things on your kayak without drilling. As for FF/GPS get what ever you like and can afford. The biggest and best you can afford is what I recommend.

Another option would be the Navionics app and run the Vexilar sonar phone for the depth. It allows you to run a split screen with the Navionics app with the depth reading from the Vexilar unit...pretty sweet and a lot cheaper, check it out, perfect for smaller water craft...http://www.sonarphone.mobi/