No food is allowed off the ship that is not in sealed, commercial packaging. So no cookies from the buffet. But you could take off one of the small containers of cereal available at breakfast at the buffet, as long as you didn't open it before exiting the ship.
This.
We have done this.
A couple points:
You MUST DECLARE to US Customs ALL FOOD. Every single bit of it. Being caught with food you did not declare, as another poster pointed out, has fines in the tens of thousands of dollars. I am not entirely sure what they do if you declare a food and they decide it is not allowed to be imported, beyond they confiscate it; perhaps another poster had info on that.
After doing a LOT of reading of the CBP website, I had contacted US CBP by email and phone before our cruise to verify that commercially packaged,
processed (not "fresh" or "raw"), unopened food could be brought into the US. Now, in our case, the food we had was all originally purchased in Canada or the US, **and I brought my receipts with me** to prove that, in case it was needed.
FWIW -- I found US CBP very helpful and recommend contacting them in advance if cruisers have questions. Their website also has quite a lot of information.
None of my food included meats, as I found another way to deal with my return-flight meal, but I had asked about meats as well: foods (in my case it was a US-origin freeze-dried backpacking meal, commercially packaged and unopened) with meat in them have lots of special rules and those that origin in most countries will NOT be allowed in. The safest bet is to NOT try bringing anything containing meat into the country. CBP had explained to me that for my backpacking meals, since they have the "USDA inspected" logo on the package they would be ok (plus, I had receipts showing purchase in the US). We decided to make our lives MUCH less complicated by leaving that meat-containing-food in the package that went from WL to BC and waited for us for our last nights after the cruise
As other posters have noted, fruits and vegetables are the other big problems for importing, especially fruits. While with many meats I believe they are mostly concerned with disease that can affect humans, with fruits and vegetables it is mostly pests and plant disease that could be spread to the US and devastate local crops. It has happened before, and the US is thus very stringent about importation of food.
Our last trip experience:
We had some
commercially packaged,
still sealed food with us (I have food allergies so bring some foods with me onto the ship; I had some left). For the original poster, those little cereal boxes/bowls, as long as they have not been opened, fall in this category (Kellogg's packages it, it is a processed food, and it is sealed).
I had all the food in my carry-on bag so it was easier to access for Customs. I was prepared to have to surrender any or all of the foods we had if Customs decided they could not be imported.
On the US Customs blue and white form you fill out I answered yes to the appropriate "do you have any..." food questions. I then wrote in above the question what we had (cookies, tea, snack bar, apple sauce, juice).
When we spoke with the US Customs person he asked us what foods we had. We told him. Everything. He said hold on a moment, I need to get someone else to speak with you. He had a uniformed Agriculture inspector come over, who asked what we had. We told him. He said we were free to go. It added maybe two minutes to our Customs clearing experience.
We have done this twice and had the same experience each time. YMMV.
SW