For the gpsmap 78sc? Don't bother.
I love the 78sc and bought a few of them so I'll have one when they discontinue them and I don't want a newer one. Charts
are mostly unusable on it. The screen
is too small and too low resolution. Get Navionics
on your phone
instead. Ten thousand times better, and cheaper.
Also, I did buy one chart for it quite a while ago. Garmin's process for authorizing a chart and loading into the unit is just about impossible. After working with support for a week, they gave me a refund.
I will load routes into the 78sc and can follow them, get trip data, and I even use it to run my autopilot
sometimes. But for actually looking at a chart, I use something else.