Let me add one more trick, one that seems to work for some of my dive group:
Grab a fresh whole lime--usually not to hard to find at a dive resort, just 
ask the bartender for one the night before--and stick it in your dive bag. 
When you feel queasy, pull out the lime, scratch the rind, and sniff it. Try 
it, it often works.
Naturally, do all the other things, too: avoid gasoline and exhaust fumes, 
don't eat a large greasy breakfast, stare at the horizon, try a scopolamine 
patch, try to be first in the water and last back aboard (this is easy, since 
the other divers will cooperate gladly if they know the alternative).
--- MacWoof 1.5.3
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