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