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