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