You're covering far too little area running submerged...
Spend a minute tracking the plane*). Is he coming too close, go to radardepth or - if very close - go deep.
Start stopwatch, give him 3 minutes to come in and some 4-5 minutes to leave, go back to radar depth.
If all clear surface and continue search...
*) if detected range is some 10-12 miles, by all means: crash dive! Then it's more than probably one of those tiny Zeros...
Distances are of course dependable of weather and your speed...