Depending on how large an area you want to cover, you could download the free NOAA charts and turn them into "custom maps". I really know nothing about marine charts, but you can scan paper maps or download scans of them and make custom maps. This can either be very simple, or else challenging, depending on your needs. Here's garmin's simple tutorial that works fine for small maps. For larger ones you would want some other software to help.
http://www.garmin.com/us/products/onthetrail/custommaps
The size map you can make is limited by the Garmin firmware. Again, I don't know anything about marine maps, but if the resolution is similar to a USGS 24k topo, the largest area you could cover would be about 20 miles x 20 miles (or anything that is about 400 square miles).