I checked the Velvet Drive Installation Manual, Service Manual, and Owner's Manual (from 82, 85, and 86, I think). All three say (sometimes in different words or tables):
Dexron II, Type F, and other hydraulic transmission fluids which meet the Detroit Diesel Allison Type C3 specifications are recommended for use in all Velvet Drive marine transmissions.
They also say that multi-viscosity oil is not acceptable and that 30 or 40 weight gear oil is only acceptable if RPM does not exceed 3000.
I can't think of a reason for your Mercruiser-powered boat to stray from the Borg Warner specifications.
I see on the Mobil website that:
Mobilfluid 424 recommended for Allison C-4 spec for agricultural uses only. It also says that it is suitable for commercial transmissions requiring Dexron and Type-F but not to use in passenger car transmissions. Who knows why? The one good feature it lists is resistance to corrosion protection in wet and humid environments. Anyway, I'm guessing 424 replaced 423 and C-4 replaced C-3.
Mobil 1 ATF, Multi-Purpose ATF, ATF Type-F, and Mercon V Synthetic Blend ATF all meet Dexron III, Dexron II, Type-F, and/or Allison C-4 specs.
So, it seems to me like you probably be fine with the 424 or just about any ATF you could find. I just wonder why the 424 said that it was recommended for (but not meeting) the Allison C-4 spec only for agricultural applications.