Challenges are there to clean the oceans and the players would have to complete them. By completing them you can earn rewards such as droplets, coins, and XP. You will see the challenges on the upper left part of your screen with a yellow ribbon icon.
Once you open the Challenges icon, it will show you the list of challenges that you have to complete and their corresponding rewards. For example, the challenge is "Sell any Plastics worth of 10k cash" to complete this, first, you have to go to your Resources and then check how much cash your Plastics Resources have. If there's anything that already has 10k or more cash for example the Bags, click it and choose to sell for 10k. Lastly, go back to the Challenges icon and claim your reward.
Collect 15 Bottles
Unlock Gulf of Mexico
Have at least 15 upgrades done on Caribean Sea
Increase organization value by 100%
Research Sonar 1
Have at least 2 Crew Members assigned