The bulk modulus of water is ~2.2 GPa, the density variation would be small at reasonable depths, so the object would need densities near 1. You'd need to include the bulk modulus of the object in question too as it will become denser from the hydrostatic pressure. For solid metals (hundreds of GPa) you may be able to ignore the variation. Temperature is a major variable too, in short this calculation is nontrivial for a precise solution.
I video spoulette burns with my camera and extract the timing from the audio track using spectrogram software. If you measure and weigh the grain carefully before you can achieve very repeatable results.
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