I currently weigh 85kgs. And 5'8". My upper portion is quite good in shape but I have a loose n' big tummy. What exercise will be good for me to tighten my tummy skin and at least to have a flat tummy. Will the stretch marks on my waist band shrink n' disappear after exercise? In what way swimming will help?

Uh, I don't know any exercises but I do know that the only way to make stretch marks disappear is by using prescription creams like Retin-A.

Yes, but unfortunately stretch marks, as I'm sure you know, are tearing of the dermis and they are in fact permanent.
The only solutions are "artificial" ones.
You can use prescriptions, store bought creams, or go to the dermatologist to go a laser treatment. None of these will completely make them go away though.

Stretch Marks home remedies - reduce stretch marks using natural techniques (http://www.homemademedicine.com/home-remedies-stretch-marks.html)

There are many expensive creames on the market -- hope you find something here

Home Remedies for Stretch Marks #1: It is very important to exercise in order to get rid of stretch marks; toning your muscles helps your skin to firm thus preventing stretch marks.

Home Remedies for Stretch Marks #2: Make sure in your diet you are getting plenty of protein and foods rich in Vitamin C and Vitamin E, they promote good tissue growth.

Home Remedies for Stretch Marks #3: Massaging your body with olive oil or Vitamin E may help.

Home Remedies for Stretch Marks #4: You can also try this homemade recipe: Mix one ounce of carrier oil ( try avocado, sweet almond, jojoba, they are the best ) with seven drops of lavender and five drops of chamomile.

Home Remedies for Stretch Marks #5: Apply cocoa butter and/ or elastin cream though out the body as directed on label. These are very good for stretch marks.

Home Remedies for Stretch Marks #6: Here is another good recipe:
1/2 cup virgin olive oil.
1/4 cup aloe vera gel.
Liquid from 6 capsules of Vitamin E.
Liquid from 4 capsules of Vitamin A.
Mix all the ingredients together in a blender. After that pour the mixture into a jar and store it in the fridge.
Apply the oil externally all over the places where the stretch marks commonly appear ( abdomen, hips,
Thighs and breasts ). If you do this consistently every day, you may prevent stretch marks

As far as swimming, it's a GREAT cardio workout and will definitely help you tone down on your body fat.

As far as your lower abs, try doing leg lifts. Lay flat on your back, and lift your legs 6 inches off the ground, then go up so your body makes an L. go down to 6 inches above the ground (not letting your legs actually touch the ground). Repeat.

If you can do this well enough to do about 40 - 50, try holding a medicine ball (5 - 10 lbs) between your feet and go at it.