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