Is it normal to fear vastness?

I have the exact same thing with space pretty much.

About half way between San Francisco and Hawaii, we ran out of wind and I thought I'd take advantage of the super calm conditions and take a swim. The ocean there is about 5 MILES deep and a lot of thoughts of what might be lurking below me were running through my head, after all, I had just gone from the top of the food chain as human aboard a boat to the bottom; a tiny creature swimming in the middle of the Pacific Ocean.
At about 100 feet from the boat, I opened my eyes underwater and there below me was a huge (I mean huge) shadow!
Those watching me from the boat later said that no Olympic swimmer had anything on me as I swam back to the boat in terror.
As I neared the boat, I once again looked down to see if the behemoth below had noticed me, only to realize that what I had seen was the shadow of the boat!
I felt a fool, but to this day, I do not like swimming if I can't see the bottom.

I think it's pretty normal. It's like the point of no return. Your story makes me think of being at a large outdoor concert, close to the stage, music so loud it's in my brain, looking back knowing there is no way I'm getting out of there. It gives me a brief feeling of panic, but you've just got to embrace it and see the beauty in it. Then you can feel how you should, like you are a tiny part of something big.

I have that feeling with the ocean but the vastness of space actually brings about an inner peace, go figure.

Go scuba diving offshore in some nice clear water during the day.

Bring your mandatory dive buddy so you won't be alone. You can go in groups even, with guides armed for the occasional sensationalized shark.

Agoraphobia - the fear of open spaces
Hydrophobia - the fear of water
Agdrophobia - fear of open spaces in water maybe?
Nah I think it has a different name but you get the gist, there is a name for all fears and phobias! However fears and phobias are different, they are sometimes misplaced and thought of as the same thing, but a fear is irrational, for example 'I fear dust' phobias however are somewhat still irrational but occur due to bad previous experiences! For example you are willing to go swimming in the sea knowing if you dive and it is empty and wide you will become scared so yes this is a fear, only way to overcome it is to dive with lots of people, not many people, no people but animals, no people few animals, no peopl no animals, and continue to progress but only when your ready! My personal example was I used to be afraid of the deep end of swimming pools however each time I'd go in the pool I'd try to get deeper, my fear arose from not being able to tread water until my current girlfriend of 3 years explainedand demonstrated how easy it was and I eventually worked my way up to the deep end which in the end turned out to be irrational also because life guards patrolled the pools regularly!

I`m creeped out by deep water too, I`ll only swim in swimming pools, not in the sea.

Oh noes you is swimming in mucky waters get out of there before its too late.

It sounds like you have a combination of Agoraphobia and Thalassophobia.

I don't swim in lakes because there is a risk of getting a brain parasite that can kill you within a week or two. A kid who was surfing at the beach down the street made the local news and had to be life flighted to the mainland after getting bitten by a sand shark. I came too hard and cried once.

I love it.