Making it only water resistant doesn't make any sense. They encapsulated all the circuitry, effectively protecting it from water, dust or any weather element. So even if water got inside the case, the circuits shouldn't be exposed.
Additionally, they put everything: S1, sensors, speakers (which Apple already said it's water-resistant) inside an unibody case sealed at every point (the display on top, and the sensors on the bottom). Given how easy is to seal the joint between the sapphire cover and the case beneath it, I would assume that those joints are water-proof sealed.
There isn't any connector on the watch, not even for charging it, so the only thing left is the digital crown and a button. If Apple sealed those, water won't enter inside the case. So why wouldn't Apple do it? It's not difficult to do, neither expensive, and even if the seal can't be perfect, everything inside is also protected against water!
Apparently all the work has been done, why would they throw it away? I'd bet it is AT LEAST water-proof for 30 minutes of swimming (or showering), though I don't see a reason why it couldn't be water-proof up to 10m or more.