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