I don't know abt you guys but i'm using olympus 720sw with under water casing (up to 40M depth). I brought this cam around last year at S'pore expo for SG999. It work wonderful for me as this cam already a waterproof itself, no danger of the casing leaking to caused any damages to the cam. So no need for double seal.

For new starter, either you buy a casing for your existing cam or you get a very good one because you won't want to change your cam and casing every 1-2 yrs. Which ever cam you going to get, it need to support your needs for at least 2-3 yrs.

My advise to take underwater pic, is better if you did more than 10 dives at least. This is to get your buyoancy control much better so that you don't go bang to the corals (we not worry abt u getting hurt but the damages to the reefs). IMHO, Diving is still an extreme sport, so is better that you are comfortable with the equipment and more experience before taking up something extra.

Remember, once you start taking photos, just the basic setup will never be enough. You will upgrade the cam with additional stuffs like strob, lens, etc.. even the extended arm cost more than the cam or the flash.. When you see other pro pic and compare with your one, you just need to buy the extra stuffs for it...it will slowly poison you to buy and buy and buy... I already dying bcoz of this... keep away if you can.....