Here is the full list of all 51 Summer in Mara achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Complete all the quests from Napopo.
Build all the main structures on Home Island.
Pay the mortgage on your boat.
Defeat the Elit.
Recover all the chrystalis from the Door.
Complete all the quests from Pilly, Milly & Tilly.
Use the recycle bin 100 times.
Complete all the quests from Litio.
Complete all the quests from Akaji.
Complete all the quests from Blu.
Sing the old Navy song on its base.
Beat Ereti in a race.
Complete all the quests from Ecco.
Complete all the quests from Rigatta.
Complete all the quests from Aquila.
Complete all the quests from Mayo.
Complete all the quests from Taima.
Complete all the quests from Awan.
Get all the Koa's outfits.
Complete all the quests from Onzo.
Visit all the islands from Mara.
Catch one fish of every type.
Unlock all the crops on Home Island.
Complete all the quests from Noho.
Get all the Koa's backpacks.
Get the boat to max level.
Get one farm animal of each type for your island.
Cook 50 dishes.
Complete all the quests from Brram.
Complete all the quests from Taka.
Meet Dunna.
Cook 30 dishes.
Sell objects for a total of 3000 coins.
Cook 15 dishes.
Craft 60 objects.
Unlock the mine.
Complete all the quests from Mûn.
Sell objects for a total of 2000 coins.
Sell objects for a total of 1000 coins.
Carry 99 oranges in the backpack.
Craft 40 objects.
Read all the messages on Puni Cave.
Ask Caleb to take you back to Qälis.
The sea is dangerous, next time get back to your boat before getting tired.
Meet Saimi.
Craft 20 objects.
Collect 90 objects.
Remember...
Wake up after the tutorial.
Collect 60 objects.
Collect 30 objects.