Hard to tell you when to come, but here are some facts to help you to decide:
1. We have only 2 seasons: rainy season from May to December and dry season from January to March.
2. During rainy season humidity is high (above 80%), October and November is when it rains the most.
3. Temperatures are around 30C to 33C during the daylight all year around and about 23C at night.
I believe then the best time to come is during dry season, but you can go to beaches, fishing or site seeing anytime.
If you want to know something else, it´ll be a pleasure to help you.
Bye

Dry season would do great, between december and march.

The rivers and beaches are crystal clear, a lot of sun and cool strong breeze. The offroads trails that are unwelcome in rainy season are great to explore in the dry season.