To get the Beach Sign, you need the Pool Toy rewards from the Casey & Skatie Event. Once the Event has begun, you have 3 days to level up as much as possible and collect the Pool Toy rewards, which need to be merged up to Inflatable Dolphin Level 6 and fed with Butterflies and Moths of different levels in order to be activated.

Inflatable Dolphin Level 6 can drop Flip-Flops Level 1, which you can merge up to Level 12 to create the Beach Sign.