Submersible electronics
Put your electronics inside of the rubber envelope filled with oil.  The rubber and oil will keep the water away while transmitting the pressure and temperature to the sensors.  

The oil could be silicone oil, or mineral oil, as long as it's compatible with the rubber.
The rubber could be a finger cut off from a glove.  


  The sensor will be dropped to at least 10m - 20m.


Note that the max pressure with the MS5540C can measure is 1.1 bar.  That's only about 11 meters of water.