As long as someone doesn't have to wear an SCBA, no problem. That basically rules out the entire sea going crew though, and anyone on a normal alongside duty watch, so really not practical. This one will be covered under the 'unless operational requirements require being clean shaven'.
Basic safety issue that we've done actually testing to prove with our specific SCBA. Clean shaven, people are able to do 20 minutes+ no problem (the firefighters can actually get 45 minutes out of it, but takes a lot of specific practice controlling breathing at high exertion).
With a day old stubble, dropped down to half that. Very quickly was below 5 minutes (ie essentially useless). Also, blows the seal, and allows stuff in. Positive pressure of the mask doesn't mean nothing gets in (and hot toxic smoke is also at a positive pressure), which we also demonstrated by actual testing.
For ref, we know people in real fires will get 10 minutes or sometimes less now even clean shaven, so we can't afford the drop, and no one wants to breath in hot toxic gas (even at small concentrations). Aside from carbon monoxide, most smoke is a giant toxic soup with things like hyrdogen cyanide, all kinds of acid gases etc involved.
CAF policy for SCBA is clean shaven, and that's to meet Canadian OSHA laws. RCN doesn't have authority to override that. C4 gas mask has similar requirements (for similar reasons) so it will be an operational requirement. Hopefully the RCN shows some institutional leadership and sends an accompanying message to reinforce that, as most people on ships have no idea what the relevant policies are.
Some ongoing discussions on religious accomodations to grow beards, but potentially means that person would not be able to fill most duty watch positions (which would be popular), and very few at sea positions. And in case anyone is curious, Sikh FFs etc generally just shave, and have a religious exemption, because no one wants their folks to die unnecessarily.
(And no, don't care that beards were allowed with the old Chemox, or that other navies allow it. We can always lead on basic safety.)