Altarose, sharks are way, way waaaaaay down the list of things to be concerned about in Thailand.

As others said, the chances of actually seeing a shark close to shore are slim at best so your wife can rest assured that it's perfectly safe in that regard.
Funny anecdote time...
The first time I went to Phi Phi, shortly after arriving to Thailand I went on a snorkeling tour around the islands. Having been born and raise on the shores of the River Plate I was very eager to actually being able to see anything underwater.
Anyway, the boat arrives to
/ominous music chord/ Shark Point, were they told us we could see reef sharks. As soon as the boat stopped I put on my gear and jumped in, swam to the spot and freedived 8 or so meters down to see what I could find (visibility was rather poor, 7 or 8 meters IIRC).
After the second or third dip I notice that there's not only no sharks to be found, but no snorkelers neither. I turn around to the boat and see all the tourists on the boat looking at me, from the back a voice is heard "Are there any sharks there?". After I assured them there were none to be seen then they started jumping in the water!.
Since then, over 5 years living in Thailand and 80 or so dives and lots of snorkeling, I've seen a grand total of two sharks. A whale shark at Richelieu Rock and a Zebra Shark (do I get a gold star for naming it properly?) at Koh Bon. Both perfectly harmless and gentle creatures.
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No sharks but they recently found saltwater crocodiles at Samae San near pattaya and closed the area off to divers. Still never heard any follow up news on that story.
3-meter-long crocodile captured off Samae San - Pattaya Mail - Pattaya News, Communities, Opinions and much more...
"Royal Thai Navy divers captured one of the man-eating reptiles June 3 off Koh Chuang. It was taken to the Sea Turtle Conservation Center in Sattahip where it will be prepared for relocation."
Now, we wouldn't want any wild beast to roam... wilderness like that, would we?
Not that I'd like to meet face to face with one while swimming but the way nature is (read with NYC mobster accent) taken care off here makes me despair. Like a couple years ago, when an enormous python, 7 or 8 meters long IIRC, was found in the forests of Koh Chang, it was promptly shot and eaten.