Sorry, I missed your comment.

To answer your first question, a significant amount of RCS for a capable commercial radar, not least of all because it is rounded for hydrodynamics, which is a terrible shape for minimizing RCS.

Second, I should have been clearer: the submarine stands to gain by operating their own radar rather than working from sonar alone.

Finally, submerged transits are mostly advantageous for security concerns.

Exactly, it’s a tiny radar signature, particularly compared to its actual size.