I wouldn't be too worried. If....by some strange chance you are actually at the beach and a tsunami comes....you will have some warning...people have some concept now that when the sea goes away like a severe low tide......go up in a tall building or get to high ground. Also, Phuket has a Tsunami warning system in place now...sirens and the like. Lastly, if you stay in a hotel even a block or so away from the beach, you will be fine. Many of the fatalities in the Phuket region were people still sleeping in beach bungalows and small, structurally weak hotels on Ko Phi Phi and Kao Lak...some people were on the beach and didn't know what was happening. However, big hotels, even ones right on the beach survived the wave pretty well and many guests just went to upper floors and watched the carnage, but were fine. The wave in Phuket I think was about 8 feet high, so easy to escape from by heading to a second or third floor area...as long as it is a solid building.
Secondly, Phuket is off the West coast of Thailand, which is the coast at risk of Tsunamis resulting from quakes off the west coast of Indonesia - which is where the last two came from. Islands such as Ko Samet and Koh Samui are off the east coast of Thailand and are sheltered from those potential Tsunamis. They had no effect whatsoever from the one on the day after Christmas.
Just some random things to think about. Me personally, I have no fear about that and plan to go back to Phuket in a couple of months.
"Those who would give up essential liberty to purchase a little temporary safety, deserve neither liberty nor safety." Ben Franklin
"The plural of anecdote is not data"