Somebody should really spoiler tag this thread (even though it should be pretty obvious to anyone entering, always good to cover your ass).
That's interesting. I was convinced there was another form, so I didn't venture too deep, but I did go a ways in using the fish just to be ballsy, figuring that it would be a bad idea and I'd just load if I died. I managed to get back out again okay because I didn't go too deep.
Honestly, I can't imagine how rough and annoying it would have been to get all the way down there just using the fish. How the hell did you get past the anglers? I feel like the game provided pretty adequate feedback that you weren't supposed to go down there, but obviously some people are gluttons for punishment... so you're right, something should be done to prevent that. It's a cool thing that people can do it if they want to, so I'd hate to say that it should be completely prevented, but it should somehow be made more obvious that it isn't the "proper" method.