If that doesn't work, see if he will possibly compromise for a simple blue-frosted cake, with maybe some graham cracker sand, and search around for some very realistic toy whales to decorate it with. Then he will have the toys afterward as a keepsake.
I have to say I have done some out-there cakes in my day, but this one takes the, er, cake. :O) Good luck! The only other thing I have to suggest is to get some really good pictures and just wing it. Which is what I usually do. Sometimes they were crooked or crumbling in the icing or there were other architecturally unsound issues, but no one ever seemed to noticed except when I pointed it out; the kids were always happy with the end results; and, well, no matter how they looked, they always tasted just fine.
at 11:10 PM on May. 17, 2009