[C320-list] Battery Advice

[hcreech at comcast.net]

One of my two batteries just  lost cranking power and i have replaced it this weekend.  I noticed on some sites that it recommended never to replace only one battery.  The original battery seems fine, both volts and amps.  I have a Xantex 20 charger.  These are dual purpose 1235 batteries purchased at West Marine.
A related question.  How high should I maintain the water level?

Thanks, 

Herb Creech
Cloud Chaser #606