I found this link to a review done on the new design 4.5 L V6 vs. a more traditional 5.0 GM based V8, it's a pretty interesting read. The boats used for the "side by side" comparision were a pair of SeaRay 22 Sundecks. The V6 boat out accelerates the V8 boat although the V8 eventually has a higher top speed.
In the article the Mercruiser rep mentions that the V6 was developed due to concerns that many of the commonly used GM engines may not be available to them in the future. They also don't seem interested in developing the GM "LS" 6.2 platform into a marine engine.
http://www.boats.com/reviews/new-mer...8-performance/