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