Whistle.
Task loading is a funny thing. Ever try to do anything in a real hurry under a tight time limit? Notice any fumble-factor?
My experience with snotty situations is that they occur real sudden-like, and they're a come-as-you-are proposition.
By the time you dig a Dive Alert out of your pocket, fumble around and unclip it, undo your hose, attach the sucker, etc. ..., your moment of opportunity may have passed you by and that's if you didn't drop the thing and watch it slowly sink out of reach.
If I were going to put myself into a situation where I thought there was a chance I'd need one, it would already be attached.
YMMV.