Hi @phamquandung, welcome to the forum 
Pymavlink is a Python library for interfacing with MAVLink communications. ArduSub (the autopilot firmware running on the BlueROV2) receives commands and outputs telemetry (sensor data, system state, etc) via MAVLink messages, so Pymavlink can be a convenient way of programmatically communicating with and receiving data from the vehicle.
MAVLink is not relevant to the BlueROV2’s video streaming or scanning sonar functionality though. The standard BlueROV2 camera has a H264-encoded output stream that gets sent to the surface via UDP, and can be received by programs like gstreamer, ffmpeg, VLC, and more. Blue Robotics sonar products communicate using the Ping Protocol, which there are C++ and Python library options for.
If it’s of interest I started a data-alignment project last year, but I didn’t get very far into it due to time constraints from other work.