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