wire them in parallel. did you get a charge controller to shut it off when the battery is full? That will produce 0.3 amps in full sun. Over a 10 hour day, about 3 Ah of charging.
watts = volts * amps
Batteries have a storage capacity rating in amp-hours. So, if you want to run a 12V 100W bilge pump for an hour on a charge, you need a 8.3 Ah battery. "Deep cycle" is a term for lead acid batteries that means they can be fully discharged without damaging the battery internally. Most car batters can only be cycled (completely discharged) a handful of times before the battery is ruined. Even if it is deep cycle, it's still better not to completely discharge a lead acid battery, in other words, buy some spare amp hours.
Here is a sealed lead acid battery (nothing can leak out): Amazon.com: B&B 7 AH 12V SLA Battery: Toys & Games
These weigh about 5.5 lbs. There are smaller and bigger batteries used in all sorts of toys. You don't need cranking amps, so don't get an automotive type battery.
I would suggest getting a waterproof box, a pelican box or the $7 wal-mart box. Drill a hole, put the battery in it, run the wires through, and seal the hole with silicone, glue, or anything else. If you not planning to be in saltwater, you don't have to worry to much about it. If your going to be in salt water, you need all marine grade wires and connections.
You can add a cigarette lighter and charge your cell phone, add a 4 battery tray and you can recharge AA and AAA rechargeable batteries for your GPS and flashlights.