First off welcome to the boards. Before we get started, get ready for the responses telling you to swap out to a SBC or BBC powerplant. Assuming that your mercruiser setup does not have any sort of electronic fuel management system, the mods you propose are a good start. In a bang for the buck scenario, I'd say that the 4barrel with new manifold will take you the furthest. Cost wise the 600cfm carbs are usually less than the comparable 300/390 cfm two barrels. The $120 you could spend on a fancy K&N breather will go a good ways towards purchasing a new intake manifold. When you have those parts installed, make sure you do get a flame arrestor that will breathe better. The standard Stainless Wire unit will do you fine for $60-$80 bucks. However, there is no sure way to know that these mods are going to give the 4.3 the extra umph it needs to get your kids out of the water faster. But within the bounds of your question, I would say go with the 4 barrel and manifold combo. There are tons of ways to get more power, but you can cross that bridge when you get to it. Out of curiosity, will your boat pull your kids up now? If it's just slow yanking em out, I'd say smaller mods should get you heading in the right direction. If you can't pull them up at all right now, there is a good chance that just a carb and manifold probably won't do the trick.
Good luck,
Chris www.liquidaddiction.net (http://www.liquidaddiction.net)