An underwater look (UW) is both incredibly interesting and incredibly bum clenching!
Assuming a twin screw ship, the boat has to approach from the stern (rear - stop sniggering at the bank) slightly faster than the ship. The boat is at periscope depth (PD) so the top of the fin is only a few feet below the surface. As the boat gets closer, the twin vortexes from the ship's screws cause interesting effects, which the skipper has to control. When the boat is very close to the stern of the ship the periscope is lowered then, as the boat passes the swirling vortex, it goes 'quiet'.
At this point 'knowing/guesstimating' the draft of the ship, the periscope is slowly raised - looking upward. It is now that we know if the team have done a good drill, as the twin screws, or at least one of them, can be seen. The number of blades can be counted, type of propeller etc, and, as the boat moves forward underneath the ship, any 'interesting' holes or protuberances can be viewed. During this evolution, which takes as long as it takes, the ship and the boat are literally only feet from each other - and only one knows what is going on!
All of this is filmed for future analysis. At the time, if you are on watch in the Control Room, you can watch the action on several live TV monitors....... and hear/feel the prop wash!
When the job is done, the boat just gently goes deeper, then tea and muffins all round!
Practice makes perfect!
This is the story of a real Cold War one - HMS Swiftsure taking a peek at the Soviet carrier (and cruise missile platform) Kiev.
Is practising underwater look operations against ferries really so risky for the target vessel?
I once suggested (on here) that some sort of robotic vehicle could be used for underwater look jobs - but that would be a challenging specification.