Firstly its always good to post your capx. It makes it easier for us to help you.
But what you want sounds relatively simple as long as you have a decent understand of Construct2.
In your diver image add another image point, place it where you want the bubble's to come from.
then add the event:
->Diver.spawnAnotherObject(Bubble, 1) // were spawning a bubble at image point 1 or what ever your image point is.
You can easily modifiy this.
For example you can set the seconds to: int(random(1, 2)) this will give you a whole number between 1 and 2.
or for any value: random(0.1, 2) this will give you any number between o.1 or 2