I find it simply amazing that after almost 2 years under water, it appears all the data is in tact - What an amazing piece of human engineering these black boxes are
Not underestimating the hostile environment down there, the most critical part was getting to the bottom in 1 piece, uncompromised. After that I do not believe the time span had much influence anymore, it's solid state memory. After 5 to 10 years+ it likely would have been still OK since the outer shell appears to be quite corrosion resistant.