When I run: $roslaunch freefloating_gazebo_demo g500arm5e.launch parse:=true
I get the error: AttributeError: 'str' has no attribute 'url'
How could I solve this error? The Gazebo and UWsim can be launched and the freefloating_gazebo demo works as well. Thank you for your help in advance!

Hi, sorry for another issue.
I was trying to control the robot in velocity, following the demo. I add to freefloating_demo/launch/PoseJointsCurrent.yaml:

body_velocity_setpoint:
    type: geometry_msgs/TwistStamped
    x: 
        min: -1.
        max: 1.
        to: twist.linear.x
    y: 
        min: -1.
        max: 1.
        to: twist.linear.y
    z: 
        min: -1.
        max: 1.
        to: twist.linear.z
    twist.angular.x: 0
    pitch:
        min: -1
        max: 1
        to: twist.angular.y
    yaw: 
        min: -1
        max: 1
        to: twist.angular.y

In the GUI with sliders, they all start with zero. When I switch to velocity mode:

rosservice call /g500arm5e/controllers/body_velocity_control "axes: ['x', 'y', 'z', 'yaw', 'pitch']"

The vehicle begins to go around, mostly rotating along z axis but also going up and horizontal.
I tried with controlling velocity on only 'x' (and position for other), and it works.
But, for example, when adding control velocity on y axis it moves even if the velocity given is 0

Could you clearify what you mean in the documentation when you say,

damping: the coefficients that are used to simulate hydrodynamic damping. 6 values are needed, the damping force will then be proportional to the velocity on each translation and rotation axis.

Are these just the geometry dependent coefficient of drag for all 6 directions of motion?

is it possible to have libfreefloating_gazebo_fluid.so working with gazebo without ROS ?
Like this plugin.

See ros/rosdistro#10327

Gazebo 8 with ROS lunar.
Running gazebo.launch of bluerov_ffg

[ INFO] [1508888507.186540871]: Loading freefloating_fluid plugin
[FATAL] [1508888507.186746160]: You must call ros::init() before creating the first NodeHandle

The target freefloating_gazebo_model_buoyancy does not exist, but it is mentioned in CMakeLists.txt
It can be solved by just removing it from the install call at line 63.

I made mistake in adding all of the other commits. The commit above is where a working version will still be as it accidently pulled in some broken development code.

If you don't mind, I can be a maintainer if this is too much work for you. I will be working to make this useful for other robots and also add code so it work with another model.

Currently the way the plugin gets its information about the hydrodynamic effects of the robot is by the tags. I propose changing the tags to something more generic such as and maybe even having different blocks within the tags for each specific effect if necessary.

Currently there are terms not related to buoyancy in the tags(drag coefficients) and I would even like to add my own tags for other hydrodynamics effects that I am modeling which i'll be parsing, but since they aren't related to buoyancy i'm hesitant to put them in that block.

My proposal,

<link>
    <hydro>
        <buoyancy>
            buoyancy stuff
        </buoyancy>
        <drag>
           drag stuff
        </drag>
    </hydro>
</link>

I'm going to be adding parameters for an added mass/inertia model and would be able to just add a block for that inside the block. Let me know what you think.

Hey, I'm getting this error:

CMake Error at freefloating_gazebo/CMakeLists.txt:63 (install): install TARGETS given target "freefloating_gazebo_model_buoyancy" which does not exist in this directory.

Probably something is missing