To resolve this, either run both ROS nodes normally (ros_client and ros_adapter), or alternatively disable certain scripts. Disabling the 'SensorRig' asset, which is a part of the 'NjordVessel' model, and the 'Nav Client' script on the vessel allows Unity to run simulations without ros_adapter running. Disabling the 'Force Control Manager' script on the vessel will allow for running without the ros_client node running.