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