From The New York Times:
No one needs to perform hundreds or even dozens of crunches, said Brad Schoenfeld, a professor of exercise science at Lehman College in the Bronx and an author of a newly published review article about core exercises titled “To Crunch or Not to Crunch.” And while everyone needs some basic minimum of core strength — getting up out of a chair requires a certain amount of core strength; serving a tennis ball requires more – “six or eight crunches would be plenty,” he said, “and only a few times a week. ...”
There is also a procedure in the New York Times article as to how one can do a correct sit-up without injuring lumbar or cervical discs. The abstract linked to above says crunches are not friendly to spine health.
The question is, how important is it to have a "ripped" core for swimming?
This study listed at the National Institute of Health seems to support that there comes a time when your core is strong enough:
Abstract: Development and validation of a core endurance intervention program: implications for performance in college-age rowers.
The objective of this study was to examine the effectiveness of a core endurance exercise protocol. Forty-five college-age rowers (age 21 +/- 1.0) were assigned to either a core training group [core group] (n = 25), which took part in a core endurance intervention exercise protocol, or to a control training group [control group] (n = 20), which was not given any specialized core training. Training took place 2 days per week for 8 weeks. Trunk endurance was assessed using flexion, extension, and side flexion tests, whereas a variety of functional performance measures were assessed (vertical jump, broad jump, shuttle run, 40-m sprint, overhead medicine ball throw, 2,000-m maximal rowing ergometer test). The results revealed significant improvement in the two side flexion tests for the core group (p < 0.05). Interestingly, significant differences were noted in the trunk extension test endurance times for the control group (p < 0.05), but not for the core group. No significant differences were found for any of the functional performance tests. In summary, the 8-week core endurance training program improved selected core endurance parameters in healthy young men, but the effectiveness of the core intervention on various functional performance aspects was not supported.
As for me I have never been happy with my core strength which could be the result of seeing too many "ripped abs" in billboards, magazines, beer commercials, and music videos. I suspect actually doing the sport you do will give you the abs you need unless it is running.