Q1. We are designing new underwater routing protocol and implementing it in UnetStack. How nodes can exchange their control information to other nodes through beacon?
You can use DatagramReq with to field set to 0, to share control information, using broadcast packets (beacons) to neighbouring nodes. See https://www.unetstack.net/javadoc/org/arl/unet/DatagramReq.html or https://www.unetstack.net/javadoc/index.html for more information on DatagramReq.