I didn't know the Navy does, or that it forbids moustaches on their own. Is that a recent change? Or maybe depends on rank or post? It conflicts with stories I've heard re: my uncle (who was a submarine commander in the 90's) being barred from beards but not from moustaches, and thus growing his out to ridiculous lengths at the annoyance of my aunt.

A quick web search and Queens Regulations don't seem to go well together. :)

Turns out to be more complex than I thought. "The CO may permit all personnel (except Royal Marines) to request to wear full set beards. RM personnel may wear moustaches at their discretion."

The moustache only ban seems to be Queen Victoria's doing.

It's up to the CO if the resulting beard is good enough. Uneven growth, e.g. scrappy, is unacceptable. The advised maximum time to achieve a sufficiently thorough beard is 2 weeks. lol

More here: https://www.forces.net/news/beard-royal-navy-tradition

As far as I know it goes back to the days of Drake. No idea if there's variations for submariners or COs. I know there are some variations of regs and traditions for the silent service.

I think we might be talking about two different navies. I thought the discussion was around the US Navy; looks like you're referring to the Royal Navy?

Indeed I was, and foolishly didn't put Royal Navy in the OP, though I thought I had.

Sorry for the confusion :)