This trinket has around 50% drop chance - you might just be unlucky ;)
The school of the stun effect is Nature, so the fact remains that it has a slight chance of being resisted.
The range is said to be 30yd.
Even if the item is not unique, you cannot use two without triggering the cooldown of one another.
The spawn of Prince Nazjak is rather slow, it may even be longer than 32-35 hours timer (very hard to make accurate calculations) - so be ready to wait... a lo(ooOo)ng time (it should also be taken into consideration that you are not alone to seek out the Prince, so the time to acquire Tidal Charm may take days, even weeks... be patient).
"Then the sea rumbled. The tides lowered and the two ships crashed into the reef. It's not just a reef. Something's down there...." -Captain O'Breen
There are reports of people waiting up to 36 hours for Prince Nazjak to respawn. wowhead comments
Patch 1.7.0 (13-Sep-2005): The stun from the Tidal Charm trinket no longer breaks when damage is dealt to the stunned target.