That cinematic soft "fwup-fwup-fwup" of a large sub prop element might nice with a low speed fabric-blade fan against upholstery. A small compressor with a long tube might be good for sustained bubble generation.
Warbling, sub-audio-rate LPF modulation might help make things feel aqueous, and blend elements together.
Actual hydrophones seem far too ...
What about not using a hydrophone, but just a contact mic instead? Since the sound transmission through water is not the frequency response you're seeking, perhaps using the drill or other electric motor on another surface, like metal or wood, would imply the same idea but provide a way to get the density of sound you're hoping for and you get to stay dry in ...
I'm a big fan of the pitched down pool toys. I did a bunch of that stuff here. They aren't pitched down in the library though.
But @noisejockey is correct, there will be lots of high end stuff you won't want. Some other cool elements to play with:
Scuba gear in pool. I'm sure all of those air releases will ...
A bit of feedback for those interested - I didn't actually get as much time to put into this as I'd of liked, but did get a couple of interesting recordings:
Both of these were recorded in areas of quite powerful water movement, my idea of recording ambient sound from more tranquil pools didn't ...
There are these pool toy guns that are just long water cannon tubes, but hovering them at different depths in and out of the water and also just blowing the empty air tube into different tubs makes some cool sound.
This isn't for the propeller obviously, but could be a good compliment for that water expulsion
there are three possible approaches to a problem like this:
using acoustic sources;
using electronic sources;
using electroacoustic sources;
in the case of using acoustic sources, you will need some hydrophones, so that you can properly make underwater recordings
please do the following:
find a proper water recipient or water container
put your ...
here is my rig I built for my home build hydrophones. It has 3 levels of shock mount isolation. Sits well in surf and wave action. adjustable directional pivots. Very quiet.
My home made hydrophone with active preamp