September is the peak of the wet season. The southern swell is still arriving and starting to space out, and what changes is everything on land.
The road to Santa Catalina is the specific thing to plan around. It takes longer than the map suggests in the dry season and it takes longer again after a week of September rain, and the last stretch is where a hire car becomes a question rather than a formality. Playa Venao and Cambutal are easier to reach and stay easier in the wet.
The compensation is that nobody is here. September falls between the northern summer and the dry-season season, so a coast that is never crowded becomes genuinely empty, and the accommodation on both sides of the peninsula is at its cheapest.

