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