The following instructions should be used if you are running a Ubuntu 20.04 (focal) derivative.

sudo sh -c 'echo "deb http://robosub.eecs.wsu.edu/repo/ /" > /etc/apt/sources.list.d/robosub.list'
wget http://robosub.eecs.wsu.edu/repo/repository_key -O - | sudo apt-key add -
sudo sh -c 'echo "deb http://packages.ros.org/ros2/ubuntu focal main" > /etc/apt/sources.list.d/ros2-latest.list'
wget https://raw.githubusercontent.com/ros/rosdistro/master/ros.key -O - | sudo apt-key add -
sudo sh -c 'echo "deb http://packages.osrfoundation.org/gazebo/ubuntu focal main" > /etc/apt/sources.list.d/gazebo-latest.list'
wget http://packages.osrfoundation.org/gazebo.key -O - | sudo apt-key add -
sudo sh -c 'echo "deb https://packagecloud.io/github/git-lfs/ubuntu/ xenial main" > /etc/apt/sources.list.d/github_git-lfs.list'
wget https://packagecloud.io/github/git-lfs/gpgkey -O - | sudo apt-key add -
sudo aptitude update
sudo aptitude install robosub-kinetic robosub-simulator-kinetic git-lfs
git lfs install

To configure your path to know about ROS software, run

echo "source /opt/ros/foxy/setup.bash" >> ~/.bashrc

Next, continue with the instructions on the getting start page.