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