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