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