I notice in the early episodes of the show it seems the voices of some of the actors were dubbed later, perhaps because the sound was too low or something. Especially Ginger and Gilligan. The episode where they make glue to repair the boat, Gilligan is trying to warn the others that "The glue won't hold!" and his voice sounds weird, like it was dubbed later.

But at least it was the original actors dubbing themselves.

Another reason for dubbing is to add something explanatory. In one of the color episodes, I recall a couple of lines being heard at the end as there's activity at the lagoon.

I notice in the early episodes of the show it seems the voices of some of the actors were dubbed later, perhaps because the sound was too low or something. Especially Ginger and Gilligan. The episode where they make glue to repair the boat, Gilligan is trying to warn the others that "The glue won't hold!" and his voice sounds weird, like it was dubbed later.

Yeah, it's super obvious in this episode. Maybe since it was filmed mostly outside there was a lot of ambient noise and a few of the lines got lost in wind, etc.