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