This would be a tiny, simple, oar-powered wooden rowboat. I'd row it out into the middle of the lake just for fun. Do l need a license? Does my boat need registration? Are there certain laws or etiquette regarding my behavior which l should be aware of? Where do l start looking?
Additionally, what if l wanna do this at night? Am l legally required to have lights or anything like that?

No license or registration is required in California for a rowboat that is not powered by an engine. A floatation device is needed. At night, a simple white light is what is required, preferably one with a 360 degree illumination.
The only etiquette l would recommend is to stay away from boats towing skiers. Any other conditions are the ones that concern your personal safety.