Sounds like this is definitely the result of a bad ignition module. I have an 02 X-Star and had the same problem last summer when I had 335 hours on it. I do have the 310 Indmar Engine, but it sounds exactly like what happened to mine. I was cruising down the lake at around 25 mph and suddenly it just started sputtering, so I pulled off on the throttle, and at that point it either died or I shut it off - don't really remember which. When I went to start it back up, it would turn over fine, but like you said, there was no spark. Everyone thought it was a problem with the fuel pump, but no.
I think I paid a little over $100 for a new GM ignition module from www.skidim.com
. I would also suggest checking your cap and rotor while you are at it. I went ahead and replaced these as well at the same time because they were pretty corroded at the time. Plugged in the new module, and it fired right up.
Good luck! I am not a mechanic, but I almost 100% sure that this is the problem.