At two weeks old my wife grabbed my son, we jumped in my father's 27 wellcraft and went to town. When we were parked he sat in a bouncy - when we were under way she held him in her arms.
The next weekend we spent almost the entire weekend on the CIG - he never fussed at all, and slept like a clam when the engines were running.
We haven't gone fast yet in the CIG because we don't have any ear phones yet, but as soon as we get them we will either place him in the car seat in the cockpit, or just let his mom hold him. He isn't going anywhere - if I thought the risk of ejecting people from the boat was that great I wouldn't own one (or at least I wouldn't let other people go out on it).
I think the obvious and most important thing would be to just not drive like an ass.