I think you meant to say/type that you "can't to treading" and if that is the case you need to learn or don't take open water. PADI (and prob others) require that you float or tread water for 10 minutes. Treading water is also the important skill you need to know if your BC fails and your waiting for the boat to pick you up. Swimming around doing the backstroke won't be an optimal way to help the boat find you, and will also make you invisible to other boats in the area that may run you over (yes this is if the OP doesn't have a safety sausage).
Also what do you mean, you can stay in the water for 30 minutes with a life jacket?? Can't you say in the water forever with a life jacket?? If being in the water for more than 30 minutes isn't for you, then neither is diving. Typically dive ops do two-tank dives, each one being around 50-60 minutes. If after 30 minutes you're over it and want to return to the boat, you are going to have a very unhappy buddy.
Sorry if I seem harsh, but diving isn't for everyone and it seems like too many people try to force it, and too many other people encourage it. If you don't like the water, don't know how to swim, are afraid of the water, are afraid of being under the water, or panic very easily...diving isn't for you. (I realize the OP is not all these things...more meant for all potential divers). Example, I think skydiving seems neat, but I am afraid of heights...I don't even like dramatic drops on roller coasters, so guess what...sky diving isn't for me. I have realized this and moved on.