Plugs can and will in many cases look like krap until the rings take a set, it is far better to see them quite dark then light in color. Be happy they are dark! When you get about 10 hours clean them or put in a fresh set, like mentioned above run the boat about 5 minites @ 3500 then just shut the key of without decelerating. Pull a few check color I am willing to bet the results will be different. I have been following your break in thread and did not see any mention of ring break in, which is accomplished by several hard pulls. You need several full throttle take offs, babying a new build assures an oil burner, hard pulls will not hurt anything realize the only thing other than a flat tappet cam is ring break in. All else if put together properly is ready to run to its fullest capacity. Best advice never ever baby a new build
Phil
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.