I've created a new Watson IoT Platform service but I can't connect devices to over port 1883 (following the documentation instructions).
Newly provisioned Watson IoT Platform services now mandate that devices connect over TLS (i.e., port 8883), rather than port 1883. You can change your service's security policy by going to Security > Connection Security and changing the default rule. "TLS Optional" will allow you to connect to 1883.
This change has been made because it ensures that Watson IoT Platform is secure out of the box: it is then a conscious decision by your administrator to lessen the security should you wish to connect to port 1883.