In Speed 2: Cruse Control how do they explain away why Annie (Sandra Bullock's character) isn't with Jack (Keanu Reeves's character)? It doesn't explain on Wikipedia.
Honestly... because Keanu Reeves said no.
From an interview on Jimmy Kimmel Live (quoted on Den of Geek):
"I loved working with Jan de Bont and Sandra, of course. It was just a situation in life where I got the script and I read the script and I was like 'ugggghhh’. It was about a cruise ship and I was thinking, 'a bus, a cruise ship… Speed, bus, but then a cruise ship is even slower than a bus and I was like, ‘I love you guys but I just can’t do it".
As a note, Bullock also initially tried to pass on the film according to Wikipedia but was convinced to do it with the promise of getting another project (Hope Floats) into production.
A little bit further down in the article it explains the in-universe reason:
After Reeves declined to appear in Speed 2, the screenplay was rewritten to remove his character from the story, which De Bont wanted to deal with early in the film. His absence is explained in the first scene, where Annie talks about how her relationship with Jack did not work out, and mentions her current relationship with Alex (Patric), before his character is introduced in the film.
So, if you did opt to rewatch the film, you wouldn't have to watch much of it.