Just wanted to pass some info on to you. Took me 2 years to figure out, so maybe I can save you some time. I bought the navionics basic map card. It does add more detail and is probably worth the money. However it took me 2 years to realize you can purchase a PC app from them for $25 that lets you update, edit and add info to your DF. With the app on my PC I can call up my home lake and see a lot more detail on my big PC screen than my DF screen. I marked spots that looked good and downloaded it to an SD card and when you pop it in your DF it automatically uploads all the info to your DF. Likewise WP or other data you have added on your DF will download to the SD card and you can add it to your PC map.
What I don't know is if you even need the Navionics Map card to do all this, but since I already bought it I guess it doesn't matter. You might want to check it out though. There are also updates from HB that you can download from their site and update your DF, but I have not done that yet.
SeaRay
Mark 1:17 ...I will make you fishers of men