Doing needlework or using watercolors are totally different from using epoxy sculpting materials.
Do your in-laws know about you and your wife's "taste" in food? If so, why did they choose Carnival? While no mainstream cruise line serves gourmet food (you have to go on a luxury line like Crystal or Seabourn for really great, gourmet food), there are lines other than Carnival where the food is a lot better. Just be aware that there are a wide variety of foods to choose from, including fruits, veggies and other vegetarian items. You can also speak to your head waiter about your food choices and they will do their best to accommodate your needs.
But I have to agree with others who have said you're setting yourself up for a miserable cruise. Like another poster said, there's more going on here than doing sculpting. Sounds to me like you need some major communicating with your in-laws on this so everyone can come to some understandings.