There are hacker sites where you can download street updates for your auto GPS. I didn't see anything for marine charts. I did leave an inquiry though.
This is the insanity of proprietary charts...for a set of new Garmin G2 Bluecharts for my region, I can pay slightly more and get a whole new GPS with coastal Bluecharts built in.
Insane...what the hell are these device makers thinking? I cant *WAIT* for Google to make them irrelevant with an Android Google Earth layer that is marine charts.