Yes, water won't harm a 12V battery or cause electrocution. The battery does however need to be a sealed battery so it doesn't get any water in it.
Up to 300lbs
It weigh's 47 lbs without motor and battery.
Yes it is very safe, in the case of a tube failure and it looses air, the body will float. Always wear an appropriate life preserver for your safety.
The GoBoat tube is made of a 1 Mil thick PVC tube. While it is a thick tube it is not puncture proof. In case of a puncture a provided patch kit comes with every GoBoat. If you lose the patch kit, we can provide you with another or you can use an appropriate patch kit normally used for inflatable kayaks.
Since the GoBoat is a motorized watercraft it may need to be registered in your state. You will need to check your local boating regulations if you plan on using it on a lake or other state body of water. All the appropriate paperwork is with your packet shipped with the GoBoat..
For the simple reason batteries are heavy and expensive to ship and are readily available at any auto parts store, automotive section at your local big box store and many others.