The DHARMA symbol http://home.blarg.net/~wayule/graphics/lost_canteen_shark.jpg
on Ethan's canteen (apparently the same one that was on the shark fin) raises some fascinating new questions:

1. Everything else in the medical hatch, down to the actor's glue, had the intertwined snakes (Caduceus/Asclepius) DHARMA symbol. The fact that Ethan's canteen did not indicates that the canteen--with its "bitter" liquid--did not come from the medical hatch. Apparently the contents of the canteen are not for medical purposes.

2. Recall Locke's mysterious acquisition during Season 1 of the substance he placed on Boone's head that made Boone hallucinate. This was during the same time frame that Locke was hunting with Ethan. Perhaps the liquid in the canteen Ethan fed to Clare also contains the hallucinogenic substance???

3. Given the connections that the other DHARMA symbols have with mythology, does the new symbol, perhaps, represent Orion's belt? If so, can we speculate as to what's going on inside the Orion hatch?

There is a post about hatch symbols and the Apollo constellations in General Theories. All three of the hatches match up, three more to go. You might want to check it out (do a search for Apollo constellations, that should find it)