A moustache or any facial hair can be a serious problem if you are a diver. The problem generally lies with the sealing of a mask to the face. Now if you are a person that believes shaving is absolutely not an option then there are a couple tips for diving with a mustache.
Divers Cut: The divers cut is a simple solution of shaving the top 8th of the mustache under the nose in order to give the mask some skin to seal agains. Because this is a way to trim the mustache unfortunately this look does not suit every diver, but it is a good solution for a leaky mask when you have a mustache.
Chapstick, Vaseline, Silicone: For those that are unwilling to trim their mustache and sport the divers cut, this is another option. All of these are viable options for getting a mask to seal, they will generally leave a residue on the mask and mustache after the dive. Some divers claim that some of these options can degrade the silicone but i have been unable to find any evidence of this. This option does work as well as the divers cut but requires a diver to remember to bring their chosen mustache sealant. Note: if you do choose to use silicone my recommendation is silicone grease used for diving related lubrication. Most divers will have some in their save a dive kit.
O Natural: This is probably the least appealing option, fortunately there is a natural way our body can help us remedy a leaky mask. Im sure all divers have experienced the accumulation of snot at the end of the dive. This is my solution for diving with a mustache. This remedy does not solve the problem immediately and your mask will leak for the first part of the dive but once the snot builds up a bit it will seal like there is no mustache at all. Some people do find this the most unappealing solution, but every diver comes up with mucus on their face so why not use it, and you never have to remember to bring it with you.
In conclusion there is no reason to sacrifice a well established mustache for diving, there are many solutions to ensure a well sealed mask when sporting a mustache.