I think that there are some types of anxiety for which visualizing a safe space doesn't work. I've had exactly the problem you're describing. I'm peacefully floating in the water and then there's a shark, a powerful undertow, a speeding motorboat...

I need more than one trick up my sleeve for relaxation. In an agitated state where my mind goes to sharks and stuff, it's much better to go for a walk or read a book or something and just stay away from visualizations. It might be more helpful to save visualization for when you're already calm or just mildly hyper but not truly anxious.