Hi,
You could try replacing it yourself. There are loads of places on the web that sell watch batteries. Just pop the back off & take out one of the old batteries. Google the number / code on there & see who stocks them.
It should just be a direct drop in replacement, but some watches need to be "jump started" by shorting with a bit of wire between two contacts. This only usually applies to digital watches though. If you put in the new batteries & the watch doesn't immediately start going, get a thin bit of wire (a bristle from a wire brush bent in half works!) and try touching various pins / contacts around where the battery sits, checking the watch each time, & you will find that eventually it will start working again. When this is done, just pop the back on & you're good to go. Saved yourself a bit of cash an all...
PS, not sure how this would affect the seal, might knacker it, but then again might be OK...