The most important instrument is the
battery monitor. Knowing the net
battery AHrs in/out is the most vital information. Some monitors support more than one shunt so you can get information about
solar and/or
wind etc.
However, a more sophisticated
solar controller with a digital display, as Mark says, is generally the best way to detailed information about the solar output. The advantage over a simple ammeter is that you get an AHr counter for solar as well as A. In other words you can see the total output as well as the instantaneous production.
The next level is to integrate the battery
monitor and solar
regulator. This is overkill for most boats, but the communication between these two
instruments allows for a more accurate
charging algorithm.
The
cheap ebay meters are still great and can be easily modified to read voltage,current, temperature etc.