I will double the suggestion of a AH gauge type meter like the link. We have one for our house 48v battery backup system. I actually "Borrowed" it for the boat, and the info is so helpful. With reasonable certainty, you KNOW how much capacity you have, how much you have used, and at what rate. It also will tell you if you have a large ground fault, and if you turn things of and on, how much juice each electrical item on the boat uses. If your ammeter is analog you will get the instantaneous reading, but not as accurate as a digital meter.
I think the mack daddy of these is the Pentametric meter
Bogart Engineering: manufacturer of the TriMetric battery monitor which measures volts, amps and amp-hours for battery systems used in homes, RV's and boats. It does everything the link meters do, for more banks, and also can output to a computer for analysis. We don't have this, but the Xantrex we do have is a rebranded tri-metric meter from them.
Chris