You would probably not be able to turn them over even if you tried, unless you had some rope or something to give you added leverage. The MII, MI, and 12' are comfortable to stand in or even walk around in some. In the 10' and PI you can stand up, but you have to be a little careful when you whatever. Actually, just about any 12 volt battery, such as a car battery or lawn mower battery, will work. The most typical battery used with our boats is an 130amp. deep cycle battery.
Most people can easily fish an entire weekend on a fully charged battery.
We use the Coast Guard's formula for determining safe loading capacity. Thus, you would have to put about 733 pounds in the 10' before it would threaten to go straight down. The safe loading capacity is a pretty conservative number, but there are certainly times when you want all of it you can get.