This is probably way more info than anyone wants...
As pa14naz mentioned, a lead-acid battery is much happier if you only use the top 50% of its charge. You can take it lower, but life is greatly reduced. Stay in the top half and you can get thousands of charge/use cycles. Use its full capacity and you may only get a couple hundred cycles. Note: "use its full capacity" does not mean "dead". If you run the battery until dead, it may never charge again. Even if it does it will be damaged and not work as well as before.
Of course in a recreational use setting, a couple hundred cycles may be fine - that might be a couple years life and these little AGMs will age and eventually die even if they are babied. Getting a large enough battery to stay in the top 50% adds a lot of weight.
Also be sure to recharge as soon as you can. The chemical process that supplies energy produces sulphate that covers the plates. If left too long it hardens and can't be undone, reducing capacity. A day or two should be fine, though the more deeply discharged the battery is the quicker the damage.
One more note, the capacity of a battery isn't as simple as dividing amp-hour rating by time. Most batteries are rated at a 20-hour discharge rate. This means a 9AH battery can supply 9AH of power, or 0.45A continuously, for 20 hours. You can draw higher current than that but the capacity of the battery drops. The higher the current the less AH you can get out of it. The converse is also true - if you draw less than 450mA you can get more than 9AH total. (The cause of this is called Peukert's law.) The battery manufacturers usually provide a chart on the data sheet for the battery that shows capacity vs current draw.
I don't know which specific model Hobie is providing and didn't find one listed as 9AH, but here's the datasheet for their 10AH version which does specify 9AH at the 5-hour rate (meaning you could draw 1.8A for 5 hours).
http://www.remcobattery.com/dbimages/do ... m12-10.pdf
Yes... I know way too much about batteries...
I put a small off-grid solar power system together and it became yet another of my hobbies!
