Casual can also mean

Made or done without much thought or premeditation. ‘a casual remark’

Association means connection.

Therefore, when people think of surfing, they connect it with fun and sun without much thought. In other words, fun and sun come to mind. However, surfing is more than this.

Casual can also mean

Made or done without much thought or premeditation. ‘a casual remark’

Association means

A mental connection between things.

Therefore, when people think of surfing, they connect it with fun and sun without much thought. In other words, when people heard of surfing, fun and sun come to mind directly most of the time. However, according to the text, surfing is more than these.