hi spiro,
there are many drugs available to treat nausea and vomiting, a limited number also affect the middle ear and how we percieve ballance etc. (cause of seasickness)
drugs such as stemitil (prochlorperazine) can be used to treat vertigo and other such ballance disturbances and are available in injection form. stemitil is available over the counter at pharmacies in tablet form. if taken prior to sickness and fully absorbed by the gut, there is no real advantage over the injection form.
alot of these drugs can make you fairly sleepy, which couuld be dangerous out on the water.
there are also other problems with injections ie: sharps disposal.
there are a few doctors around who specialise in dive medicine who will know alot more about this subject, they might be able to help out.
if all else fails, feed your deckies tuna sandwiches, it makes great burley
cheers
dazza
disclamer: the above is intended for information only. consult your health care professional before taking any medication