San Marine State Park isn't listed on the Find a Park list at the Oregon State Parks web site. OregonLive.com says:
The undeveloped historic area north of Yachats is best accessed by the beach - just don't expect to find any indication that you've arrived.
That probably explains why I've yet to notice it in my travels in the Yachats area. Next time, I'll check this map. It seems to be just south of the (unincorporated?) community of San Marine. Across the highway from Sunset Street, and maybe extending north, on this map. It looks like Sunset St is just north of milepost 162.
If indeed the beach is the best access, the Tillicum Beach Campground is one sandy mile to the north, and Smelt Sands SRS is two miles to the south.
See pictures at Mapcarta.com, Google Image Search.