Hi NKN, unfortunately there is not much documentation about UWSim yet, just what there is on the wiki:
http://www.irs.uji.es/uwsim/wiki/
However, it comes with a set of examples that should be self explanatory, being gotoAbsolutePosition one of them. It requires a nav_msgs/Odometry topic where UWSim is publishing its current pose (via a PATToROSOdom interface), and another nav_msgs/Odometry topic where UWSim is listening for position update messages (via a ROSOdomToPAT interface).
Your scene xml file must include these interfaces and specify the topics, e.g:
<ROSOdomToPAT>
<topic> /pose_update </topic>
<vehicleName> your_vehicle_name </vehicleName>
</ROSOdomToPAT>
<PATToROSOdom>
<topic> /current_pose </topic>
<vehicleName> your_vehicle_name </vehicleName>
</PATToROSOdom>
Note that the default cirs.xml scene does not include the PATToROSOdom, so you need to add it explicitly if you want to use the gotoAbsolutePosition example.
Then, with UWSim running, you should be able to do:
rosrun UWSim gotoVehiclePose /current_pose /pose_update 10 0 0 0 0 0
Hope that helps!