dehydration

well my wife has had dehydration twice in the past 4 months. both times she has had so bad that had to give her a iv. she said she has been trying to drink plenty of water but it upsets her stomach too and makes her dizzy. she said it has been hot. she is on a us navy ship. the food on the ship upsets her stomach enough to were she has diarrhea. she has always been irregular. its either diarrhea or doesnt go for awhile. she is 30 yrs old. maybe someone could help me find out what is wrong with her. i have thought of crohns or ulcers. appreciate the help!