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