Here's what I think: I already have you in my yahoo messenger! I have got a bad stomach ache since this morning - it is written correctly, I would also say: I have had a bad stomach ache since this morning (assuming that you still have it - Present Perfect) Instead of the last sentence, I would say: "It's been eight hours since there is no water (in our place)" or "It's been eight hours since we are out of water".
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