always use rehydration tablets if I show any signs on being dehydrated.
This could be part of the problem, if what you are taking is salt tablets. They can add to the nausea. Unless you have been working very hard in very hot conditions, it is very unlikely that you have incurred sufficient losses of salt to require them, as well.
There's a very good article in this month's Scuba Diving magazine on seasickness, which goes over ways to approach it, starting with where you focus your eyes and ending with a discussion of medications.