Our youngest, CHase, was on board at 5 days. The boat is not the limiting factor, nor the baby... it is the parents. Getting on/off is the only tricky part. But you will find the gentle motion of the boat will rock your child to sleep and Chase fussed a lot less on the boat than off. Of course, I think the parents were a lot happier on the boat too so that probably played a part in it.
If you are serious about boating with a baby, let me know and I will give you a lot of thoughts and things that worked for us... and some that did not.