sometimes, after my first dive in a two tank dive, i get a little sea sick on the boat during the surface interval. once i get back in the water for the second dive i feel fine. I found that if I don't eat breakfast before going diving, i am more prone to sea sickness... it's weird, i would have thought it would be the other way around. But now that I know that, this sea sickness doesn't happen as often, i also think part of it may be psychological lol.
the only other thing, is sometimes feeling exhausted as George1224 has mentioned. I recently got nitrox certified, so i'm looking forward to that.
Remember, if you have any rash, headache, weakness, or tingling after a dive - get it checked out, make sure the dive master on the boat knows if you are on a charter, because it could be a sign of Decompression Illness which includes Decompression Sickness or Arterial Gas Embolism.