How to view multiple inboxes on Apple Watch (individually)
From the Apple Watch user box (https://manuals.info.apple.com/MANUALS/1000/MA1708/en_US/apple_watch_user_guide.pdf): "Choose which mailbox appears on Apple Watch. Open the Apple Watch app on iPhone, tap My Watch, then go to Mail > Include Mail. You can specify only one mailbox, although if you don’t choose a mailbox, you’ll see content from all inboxes."