Changing the voltage doesn't change the number of batteries, it just changes the wire size and voltage drop in the system. High voltage makes it more efficient, but doesn't change the amount of power draw, or the number of batteries needed to provide that power.
I really doubt that a 5kw or power would be enough to reliably move your boat. You are talking about the equivalent of 6.5hp, on a 14,000lbs boat that was originally specced with a 40hp engine. The original engine probably put out more than that at dead idle. I know for a fact that my 37hp westerbeak puts out 10hp at 1000rpm, which is dead idle on it. Or more than double the output you are designing to (see http://www.westerbeke.com/productbrochures/w40.pdf
Look, converting a diesel powered boat to electric is possible. But there are major trade offs when doing it, mostly involving range and available reserve capacity. To offset these disadvantages you have to install huge battery banks, switch to higher capacity battery technology, or add a large generator. Each of which comes with other major problems.
The reality is that diesel fuel is roughly 100 times more energy dense than lead acid batteries, and about 50 times as energy dense as the best (read most expensive) batteries available. So every pound of diesel fuel you replace has to be made up with by 100lbs of batteries.