It seems to me pretty likely that 3rd party programming for the Apple Watch is only likely to be supported in Swift.
It's a chance for Apple to start changing the way Cocoa APIs look.
Lots of the syntax for Swift is driven by the need to be as compatible as possible with Objective-C based cocoa.
We wouldn't have method signatures which look like
tableView(tableView: UITableView?, numberOfRowsInSection section: Int) -> Int
if it wasn't for a need to maintain this compatibility - the main oddness being the placement of the first parenthesis.
(The equivalent in Objective-C is
- (NSInteger) tableView:(UITableView *)tableView numberOfRowsInSection(NSInteger)section
I expect to see a Swift only Watch API, with a larger use of Swift only language features (especially closures), which frees the API designers from the limitation of having to design for two languages.