Would laying on hands and counting be something to consider?
If wrist/neck/groin pulses inaccessible try temporal (just behind the temples).
If you're wanting to compare rates at different pressures could you mark waypoints on your computer and correlate the two later?
Just a thought,
Chris.
Effect of immersion, submersion, and scuba diving on heart rate variability -- Schipke and Pelzer 35 (3): 174 -- British Journal of Sports Medicine Commentary -- Wilmshurst 35 (3): 180 -- British Journal of Sports Medicine