You can visit friends' reefs:

In your menu, you have the option to visit the reef of a friend. You can select random or add a username.

The point of visiting friends, aside from checking out their reef, is to tap on their items and earn rewards for

both of you. Before you jump in and start trying to tap everything, please note that you only get to tap FIVE

items per day. Soooo, choose wisely.

You only get half of what you would get in your own reef. My tip for this is to carefully edge your way around

the reef (if you accidentally brush a Fan Coral, it counts against your five...)

and find the highest yield items.

When you're checking out reefs, find one of these - Cat's Paw Coral, Red Tables, Red Christmas Trees, Bamboo Coral, Psychedelic Coral, Pipe Coral, or Yellow Small Grass.