These have been around for years.

They work on the photo-voltaic principle and are foolproof unless
damaged.
It will take a lot of square footage to bring up a seriously discharged
battery in an acceptable period of time. A small section can be useful
for keeping a reasonably well charged battery "topped off".

How does the first battery charge in your boat? Off the engine?
Assuming any sort of output from your alternator it wouldn't be that
tough to charge both batteries at the same time. Proper switching will
allow you to isolate the "starting" battery at anchor or other times
the engine might be off and thereby ensure that you won't be "dead in
the water" when you want to restart.