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