The ASA 106 Advanced Coastal Cruising certification is the certification recommended for sailors who anticipate purchasing a sailboat. The ASA describes this certification as follows:

Able to skipper and navigate a sailing vessel of approximately 30 to 50 feet in length in coastal and inland waters, in any conditions day or night.

The key skill that they train you on hands-on is to take the sailboat in to port at night. I obtained this certification with the Bay Area Sailing School as a result of the 3-day, 2-night course which involved practical in a cutter-rigged keelboat and also a written exam.