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