Greetings Straph!
Thank you for reporting your situation regarding Aspect of the Drowned.
In order for our team to properly assess the issue, could you please let us know if you are playing on an Official Server or Private Server please? In case you are playing on a Private Server, are you perhaps using mods?
On the private server, tested without mods, and still has issues.
After testing it extensively, the helmet water breathing buff turns on after I put another helmet on. It seems like an issue with the game mechanics not working correctly. While the helmet is on, sometimes I’ll get the iron lungs, but never get the water breathing.