@ Ian W Acoustics communications
Change the frequency - Remember this is not radio this is sonar a lower frequency in water has a lot more range.
Change the pulse recurrence frequency so that there is one pulse every minute or even every 3 minutes by charging a capacitor bank then discharge that for a greater power outputEncode the signal with the last position from the DFDR/GPS and the aircraft ID
You're right that VLF at 8Khz would give better range than 37Khz but not much more, certainly not in terms on NMs. Water presents just too much attenuation unless the source uses enormous power.
What of the idea that the ULB remains asleep until it is interrogated so extending battery life. , much like transponders that are used by Offshore oil and gas vessels which a give an xyz position Extra information could also be supplied easily depending on the firmware used. All this technology is already available but would imagine it would take years to get through CAA legislation.