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