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