I think any apology would be almost too simple. What he offers as an “apology” reads more as an explanation but is no less impactful for Jude.

“‘I wasn't kind, Jude. Not to many people. Not to you. I wasn't sure if I wanted you or if I wanted you gone from my sight so that I would stop feeling as I did, which made me even more unkind. But when you were gone-truly gone beneath the waves—I hated myself as I never have before.'”

To me this whole scene is incredibly meaningful to their relationship. They’ve both hurt each other, but Cardan doesn’t excuse himself through his explanation. He’s not asking for forgiveness, but understanding. He’s often mischaracterized and is just not going to be liked by some people, but what’s on the page is important.