Hydrophone sensitivity rating
Hydrophones have a weird way of specifying sensitivity. $1V/ \mu Pa$ simply means $10^6 V/Pa$ which is frankly an idiotic unit. 

You scale this by -200dB and  you get $ S = 10^{-4} V/Pa$ or $S = 0.1mV/Pa$ which is a much more reasonable representation. 

This has NOTHING to do with dBSPL which is referenced to the human threshold of hearing. There is no reference needed here, since you are using Pascals directly.