During World War 2, both SONAR and RADAR where deployed against submarines, both from planes and surface vessels. Anti-submarine warfare planes, until they built radar arrays small enough to fit on those planes, where basically large spotter planes, often seaplanes, with a high flight time. They looked for the submarines using their good ol' Mk I Eyeball and binoculars. This was the same for surface ships (they too were retrofitted with radar in the later years of WW2).
When they employed Radar against submarines, it was to detect them when they were surfaced, or later in the war as they were snorkeling, as submarines were basically just ships that could dive for a finite amount of time, and had to resurface when their batteries were depleted. So usually, submarines would surface at night or during bad weather, since spotters would never find them. Radar did help with this, as radar could pinpoint their location pretty good. However, this did not work when the vessel had been submerged.
Even if Radar waves were to hit the water surface orthogonally to minimize the huge reflection/scattering effects caused by the water surface, radar will only penetrate a couple of meters, and even if it reflects off the submarine hull then, on the way back through the waters surface it will also reflect and scatter off of that surface. The remaining radiation wouldn't be sufficient to detect anything, as it would be much less than the reflections off the waters surface.
This is also the same with radio. It's why civilian submarines are usually connected to a surface ship by cable to communicate.
So, the alternative to detect these vessels were, at first, hydrophones (basically microphones for the use in water). Sound waves travel much faster under water than they do in air too.
However, this method had also multiple drawbacks. First, you'd have to be quiet enough to actually hear the target over your own engine noise, and second, the target must be loud enough for you to be able to discern it from the background. Thirdly, water has thermal layers that reflect sound waves in the same way as water surfaces do with radiation. It got partly remedied by using an active ping, the predecessor of the sonar, ASDIC.
The thing is, both ASDIC/SONAR and RADAR depend on the property of the target to be reflective in a way you can pick up the reflection at your vessel (or deployed buoy or whatever).
You will not see anything when the waves get absorbed. You will not see anything if the reflection of sound/radar is not going back to you. If it is reflected away from you, you wont see it. Now, since its waves we are talking about, there is little chance there is literally zero getting back to you, the problem is that what is coming back has to be discernible from all the other reflections you pick up.
Now jellyfish tend to be 99% water... I am pretty sure you won't see that on a rather primitive sonar system, as the wave will probably propagate through the jellyfish. I recon this would be the same with radar, even if you were able to penetrate the waters surface and/or reach more than a couple of meters range.
No, not with mid 20th century technology.