Although it's not necessary to have a tour guide when going to the lighthouse, it's a good idea to have one take you there (ideally at sunset). Since you'll need to cross the lagoon to get there, you'll need to hire a boat, so you might as well have a guide show you the trail and relate some of the history of Chacahua.
Heading up trail.
Matt shooting video of the surf on the mainland side of Chacahua. The lagoon in the background separates the mainland from the island.
View of the island from the lighthouse. Playa Chacahua is lined with restaurants and bars. Grab a cerveza and a hammock when you get back and enjoy a different view.