Not sure about the 30m regular E, but I have the 100m Emergency Mission and have gone to pools, oceans, and even extreme water parks (closest to jumping from a chopper into water ya know...

) wearing it without ever a problem.
Personally I wouldn't hesitate wearing a modern 30m rated watch swimming. If you dive, pressure at 30m is quite intense -- can't imagine recreating such forces from swimming at the surface to worry about overcoming the seals.
Then again, I've always had problems believing the need to "interprete" WR ratings... If Casio can mass-produce virtually indestructable G-Shocks for such little money, can't imagine why Breitling's fine-precision instrument that says 30m WR wouldn't mean exactly that?
+1 I wear my Mission 24/7 and don't think about it. I've always wondered why the Ti Emergency is only 30m WR though...
That said, I'm not Bear Grylls and the deepest it ever goes is about 3m at the local 'pool.