I want to show a simple message in watch from my android application. Is it necessary to design a watch face for that ? If no, how can I do that?
It depends on how you want to do it, .net, native or web app.
there are examples on this page that should help.
Samsung Developer Program
You have to create SAP service. The communication between watch-app and Android app via SAP works with network interfaces like Bluetooth, Bluetooth LE or WiFi. Follow the link from Ron’s answer and pay attention to section File Transfer.
You need to download Samsung Accessory SDK https://developer.samsung.com/galaxy-watch-develop/sdk.html
I have already developed an Android app. Now my requirement is when my phone is connected to galaxy watch, I want to show some messages in watch when I perform certain action in my app. I don’t want specific UI for watch. Just show my message in any panel. I don’t want to create a watch application using Tizen studio.
You can try to use Push Notification https://docs.tizen.org/application/web/guides/messaging/push/
However, you still need a simple Tizen application on a watch.