Old-school metallurgy required the extra bearing tolerances. Older crank forgings (and castings to a lesser extent) had much larger variances in heat-related size changes. Since a marine application runs at much higher loads, the bearings saw typically higher temperatures. Those higher temps did two things; swell the metal and reduce the bearing tolerances, and raise the oil temps accordingly. Wider gaps and 20w50 were the solution.
Todays castings are not only strong enough to replace yesterdays forgings, but the metallurgy has improved heat expansion ranges allowing boats to use street-type specs and 10w30.
I am not an expert by any means, but I have torn down and built about 5 marine engines - mostly small block chevys and one 351 Ford. The Ford was a 1981, and it still had 4-bolt mains and a forged crank, but otherwise it had the same exact rods, pistons, heads, etc that you would find on the street. The one I'm building right now is a marine chevy 350 Vortec, and not only do the guts have the same exact casting numbers as the street version, it even uses the same carbureted intake casting number as the ZZ4 crate engine.
I'm continually shocked at how similar they are these days. With the exception of salt use in the water jackets, I have no problems interchanging them; street/boat.