Some water sensors can be connected to leads which will detect water when the two are « joined » by water. Those could potentially work if added to the drain if it is made of PVC (not metal). If made of metal, a temperature sensor should work well assuming you do not shower with room temp water. 
Do I understand that you want to ensure the light stays on while in the shower and that the fan turns on when the humidity gets to a certain level?
What I have done that works quite well for both my wife and I is:
Bathroom has a 4-in-1 sensor - light needs to be turned on manually. Will turn off automatically after no motion for 4 minutes. (Using @bertabcd1234’ lights on motion plus app). I prefer to get ready in the dark - helps me fall asleep afterwards. My wife, not so much.
Shower - light needs to be turned on manually. Will turn off automatically after no motion in the bathroom for something like 20 minutes (needs to be longer than the longer we will typically shower for).
Humidity - I compare the humidity levels from the bathroom and another 4-in-1 and turn on the fan if the humidity is greater by a certain % in the bathroom than in the other room. There are also apps that will turn it on based on a single sensor’s humidity level.