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