The Backscatter Mini Flash is not a bad choice, as long as you are willing to work unthinkable it’s restrictions. For macro and close work it will do the job. Do not expect it to be much use for wide angle. I have one, but have not had a chance to really use it (Covid).
Another option is buying used. Lots of very affordable equipment is sold here daily.
If you feel that only one strobe will be too restrictive you might want to read Alex Mustard’s book. He stresses using “only” one strobe in all his workshops. Effective close and macro photography can easily be accomplished with only one strobe.
Enjoy the journey that we call Under Water Photography, it is addictive and very rewarding.