JSN-SR04T ultrasonic sensor use inside water tank

Hi all - I'm wanting to build a v2.0 gauge for my rainwater tanks, based around the SR04T ultrasonic sensor family. My previous v1.0 gauge was a temporary one using the HC-SR04, that required me to get a step ladder, climb up and open the tank, and then place the gauge at the top of the tank opening to take the measurement. This was a) time consuming, and b) fraught with the worry of dropping my electronics filled project box - including an arduino - into the tank… For my v2.0 model, I want to make something with a sensor permanently in the tank (which is not a good environment for the HC-SR04…), and a display module in a more accessible location outside the tank - possibly mounted in a waterproof case on the adjacent wall. With these requirements in mind, I'm looking at the JSN-SR04T, which has a separate moisture sealed sensor on the end of a 2.5m lead.
My question, to anyone that has used these, is are they suitable for vertical ranging in a water filled environment, where a drop of condensation is potentially likely to form naturally on the surface of the downward facing sensor? Obviously, I will fix the sensor into the upper surface of the tank, above the overflow level, so it won't get submerged in water even if the tank is ‘full’, however, from what I have read, these are possibly the same kind of sensors that you see embedded in the bodywork of cars, as parking sensors, and are designed to be mounted for horizontal ranging, where any surface water would naturally drain off the surface, rather than being used in a downward facing orientation.
Comments and thoughts welcomed from anyone that has any experience with these sensors, especially for this kind of application.
TIA
Grum
Further investigation has revealed a couple of things - firstly, it appears that all of these devices that rely on a single transducer, have a substantial blind spot up close, and won't give good readings below about 25-30cm range (which is a problem as the ‘full’ point of my tank is only about 8cm below the top of the tank…), and secondly, as with a lot of ebay listings, the seller is using stock photos and description, and doesn't actually know anything about what they are really selling. It looks like the stock photos are actually for a completely different unit and not the JSN-SR04T as advertised.
So - back to the drawing board… does anyone have any ideas (electronic - and cheap, as opposed to mechanical options such as sighting tubes or floats)?
Cheers
Grum
