I'm thinking it has to do with the electrolyte imbalance and higher hematocrit maybe? Dehydration will cause hypovolemia which will increase the hematocrit. Blood is more viscous and not readily flowing to the GI tract causing the muscle irritability or spasm which causes the N/V.
Another poster said that blood shunts to the GI tract causing N/V. I would think that's counter-intuitive. An example would be how blood shunts away from the GI tract during exercise to fuel the skeletal muscles. That's why you can get nauseous because less blood is flowing to that area and fluid is being lost from sweating.
For you other question, yes, you need an order for ice or hot packs from what I understand. Cold packs will cause vasoconstriction and reduce swelling so you'd want to use it on post-op wounds, sprains, etc. A post-op hip replacement comes to mind lol. On the other hand, heat will cause vasodilation and increase blood flow to the area. That would be a good treatment for stiff joints, sore muscles, etc.