Zinc and phosphorus requirements are directly related to valve spring pressure and cam ramp rate. Marine engines have low lift, slow ramps and soft valve springs, and do not turn high RPM. There's no need to run an additive except for cam breakin or if you are running an aggressive cam (ramp rates, not duration) and strong valve springs.
Personally I use Rotella T6 in the 454 in my boat. I get it in 2.5gal jugs for my truck and it's easier to just get one oil that works well for everything in the fleet and is a reasonable price. zinc/phos levels are a bit higher in T6 than in typical oil today but not what they were in proper oil a decade ago. It's good enough for my engines, even the 460 with a fairly rampy cam and stiff springs.
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