Northsider,
You have also found that the capacity 12AH and 7AH comes from lead so the weight of a 12AH will be (almost exactly) 12/7ths more than a 7AH battery!
Please tell us what the motors are - there are many very knowledgeable folk who will know them and be able to give you chapter and verse. Also it is actually the prop(s) that take the current - ideally it helps to know the prop size and boat speed as well to estimate the actual current draw "on the water"
As an aside, I and many others are dyslexic, and have a lot of trouble reading capitals, so would you please try and post in lower case. Some people also consider it to be the equivalent of shouting
regards
andrew