You must first find the HM Rock Climb in the blizzard next to the house of the man who said he lost an Item. Its not a hidden object so dont worry. Then you must beat Candice the Ice type gym leader and get you 7th badge. This will allow you to use the HM and Rock Climb up to the lake. Enter through the trees like you did with the other two lakes.
To get there you need strength fly to solceoan town(sorry i don't know how to spell it). Then, go south there you will find a tower at the top floor of the tower is theres two old ladys, the one on the left will give you the HM Strength. Then with the HM got to Mt. Coronet(Probably spelt that wrong too) then use Strength to move the silver rock out of the way. Then make your way through the mountain until you get out of the cave which will bring you to a place covered in snow.Make your way through the snowy land and along the way find the HM Rock Climb. Then go to the city and beat the 7th Gym leader Candice. Now you can use Rock Climb to enter the lake.