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