Because there’s no other use for boiling water than dungeness crab? 
I’d be more likely to suspect the short list included more than one type of shellfish or vegetables that required boiling, and they hedged their bets with the water.
Given the time constraints, that’s just good preparation. If I were on the show, not only would I have water boiling (which could be used not just for crab or lobster but also for vegetables or grains) but I’d also preheat the ovens and the fry-o-lator, maybe some charcoal or smoking chips, maybe oil in a saute pan, that sort of thing. If the ingredient had turned out to be one of the other alternatives — bell peppers, mussels, pigeon thumbs, used bandages, whatever — you’d notice the cooking method/device that was preset for that item instead of the boiling water that was used for the crab. For example, if I knew that there was a chance the ingredient would be several varieties of rice, I would heat up a big pot so I could dump in a bunch of butter right at the bell to begin melting it for risotto, and not waste time while the vessel gets hot. Bottom line, they’d rather not have to wait for the tools to catch up when they’re done with some stage of peeling-and-chopping prep work.
I don’t have any particular investment in being ‘right’ here, but for further evidence I offer:
Challenger Walter Scheib brought pre-prepared crab stock to the battle.
Iron Chef Cora brought those small crabs (forget what they’re called) to complement the dungeness crabs.
There are lots of other examples where a chef brought a very specialized ingredient to use with the secret ingredient. I guess ‘hedging thier bets’ is still a technically-possible explanation, but unlikely in my opnion.
Also note that the sous-chefs always know exactly what will be prepared, without any kind of ‘team meeting’ or notes, unless these are very deliberately kept off-camera.
Also note that they often bring very specialized serving dishes.
I think there’s probably cases where the team has completely rehearsed their whole ‘battle’ in private beforehand. Doesn’t diminish my enjoyment of the show at all.
For the record, these were HUGE (~5 gal) kettles that you would never use for vegetables or grains, and that you never see present as ‘good preparation’ in other battles.
Now I’m sounding defensive and it’s really not that important to me. Guess I’ve thought about it a lot as I’ve watched.