I've used the 176C and the upsides were
1)Ability to remove and take it with you leaving the RIB for security purposes,
2)No antenna cables running from front to back of boat and other advantages mentioned like being able to upload maps and the chartplotter function etc.
1) Screen not the easiest to see once the backlight went out and as Philip mentioned, limited adjustability of the backlight.
I now have the 276C which is even better!
1) Battery pack supplied so if you don't want to hardwire it you still don't have to replenish it with new batteries.
2) Backlight functions are much more user friendly with a night/day function where the screen changes colour for easier reading. The screen is actuallly easier to read even than the 176C without the backlight function on.
3)Can be used in the car and has a voice command function (not included)to tell you where to go... (My last navigator did this as well, but the directions were always the shortest route to hell
The issue of size hasn't been a problem for me as the data size can be changed to extra large font for easy reading (If necessary)and the zoom function on the map is ample for me to follow close up even when moving.
From an availability standpoint, I don't know when they are available, but if you give Garmin a call, I'm sure they'll have a good idea.
In a nutshell, I'm very happy with it!
Hope that helps!!