The thing about crunches, which seems to have been lost lately, is that they were developed to NOT train your core.
This is so atleast according to Paul Wade's book Convict Conditioning, which btw is a real good book that I can recommend anyone interested in body weight exercises to read.
What he sais is that when bodybuilders started to pop steroids back in the sixties they were afraid of a syndrom called 'roid gut', which is caused from steroids not only making your muscles
grow but also maings your inner organs grow freakishly large.
Here's an example picture showing what it might look like, or just try googling it if the link doesn't work: http://forum.bodybuilding.com/attachmen ... 1168902832
To avoid this they shied away from any exercise that could possibly add any extra girth to their core, and thus any functional ones, and instead started doing crunches just to
that only toned in a little extra definition on their six pack without adding any real core strength.
That said you might think they sound good if its definition your after, remember that visible abs are mostly about diet and not training.
Ones you get your carbs in check and drop a little body fat you will see that your unintentional core work from general exercising will give you definiton enough to be proud of at the beach.