Of course you're absolutely right and even without a helmet I wouldn't go scuba diving somewhere without at least somebody near who might be able to help if anything goes wrong.
But then... you were tripping with a self-build helmet underwater! Who knows really how important safety was for you at that moment? But I must say, you really have a chill cousin supporting you with that idea :D
I haven't dived for way too long... gonna finish my studies soon and afterwards I'll hopefully be diving again. Just seeing the pictures of the wolf eel makes me wanna get my equipment and jump into the next lake already..