Substitute no... great time... yes. We are big WDW fans, infact we try to visit atleast twice a month if not more, of course living about an hour away helps but if we lived out of state I'd have to make atleast a few trips a year. The parks are fun but they can be a tiring time as well, how old are the members in your group?
The cruise is wonderful and relaxing and you still get the Disney Magic the parks provide with a whole new twist. The characters are there, the service is wonderful and of course there is Castaway which is Disney little piece of paradise. Of course you won't be overwhelmed with Disney either, there are adult only areas that are not "Disney" fied but rather very well themed.
I wouldn't say one is the substitute for the other but if you've done the parks before I would try the cruise, of course don't blame me if you become addicted. =o)