This from globalsecurity.org and copied from another forum:
Initial indications are that Diego Garcia was not affected by the Andaman Tsunami of 26 December 2004. It is located south of the tip of India, well with in range of what the tsunami, with a max elevation of 22 and an average elevation of only 4 feet. Civilians monitoring shortwave radio reported on rec.radio.shortwave that a female operator, in answer to a query from an aircraft after giving weather information, reported no ill effects from the earthquake. So everything appears well on Diego Garcia.
Cheers! M2