Books like The Adventurous Four

When a trip at Little Island for the weekend ends in them being shipwrecked, it is at first exciting for the Adventurous Four. But imagine their surprise when they discover the hidden harbour is full of enemy submarines! They investigate more closely so as to get back to the mainland and expose this terrible enemy. But a forgotten camera leads to their capture. How can they outsmart the enemy and escape? And is there any way of getting off the island...?