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