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