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