I'm thinking of taking the single vehicle ferry across the Sierpe river (from Sierpe), then on to Rincon. We are staying in Sierpe one night because of fishing trip out of there early one morning.
Is that the road you are thinking of?
We took this road about 4 years ago in a small rented 4x4. It was steep at points, but was maintained and populated by locals. It shows as a road on the maps and lived up to this....unlike some roads in CR. There are some great views of the peninsula to the west and mountains to the east weather permitting. It is not something I would recommend to people inexperienced with narrow dirt/rock roads, but with that said, we never shifted into 4 wheel drive and never had to back up to allow oncoming traffic ... things we have had to do elsewhere.
For the record ... there is another road a few miles south of where this one comes out on the main highway. It shows as a trek, but is in fact a decent road and goes all the way out to Drake Bay. Small delivery trucks and local buses use it regularly in the dry season. There are three small creek crossings and one small river crossing (near DB) all of which were managed without four wheel drive. The river was about 14" deep at the time we crossed, so the vehicle's clearance came into play. This last crossing is not one I would recommend in the rainy season. At all times, until you have done them once, it is probably worth waiting to see how these sorts of crossings are navigated by locals before diving in headlong ... especially if you don't have the full insurance package on your rental (which may be voided because these backroads are not always shown as roads).