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