I would not worry about water sealing. It would not help much to have an L lens if the camera body is not weather sealed too, and the Rebel is not. But they won't break just because of a few rain drops or some spray from the breaking waves (at least mine didn't).
On a Rebel, I think you will get much more out of the 17-55 than out of the 24-70. 24mm is not really wide for a general purpose lens on a crop camera, and the 17-55 is optically at least as good if not better. There is a reason why Brian (the TDP Brian) recommends it for crop cameras (see here http://www.the-digital-picture.com/C...pose-Lens.aspx):
The Canon EF-S 17-55mm f/2.8 IS USM Lens and Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens were introduced after both of the aforementioned L lenses and I now highly recommend them to APS-C/1.6x body users.
Adding a 70-200 is a great idea IMHO, but I would not buy it before the general purpose lens.

Arnt