You might investigate if a submersible pump can be repurposed as a lower-voltage motor. I don't know if any are built for 12V/24V DC, but maybe worth a look.
A 3000-RPM motor that runs on 120V, might run at 1200-RPMs at 48V. Many "universal" motors run fine on either AC or DC...
If your needs are for a motor that will be immersed in liquid, heat would be MUCH less of an issue, so it should be fairly easy to fully pot an existing motor.
If yes, what is the ideal non-waterproof brushless motor for the job you need? Watts, volts, size, RPMs?