How do I change the battery in my Swatch diver's watch?

I bought a (pardon the pun) secondhand Swatch diver's watch fairly recently. Therefore there are no instructions as to how to change the battery, which is plainly dead or missing. But I dont know how to replace it. On the back there is a small circular cover with 3 tiny locking lugs on, and this just screws off. However this reveals nothing but what seems to be a plain layer of rubber with no obvious way to get it off. I think the battery may belong under this, but I dont know what to do next. Pull the rubber layer off? Has anyone else encountered this -it would be a shame to do something wrong and ruin the whole thing. Thanks.