Random Riddle: 2 Different Names - Common Sense Evaluation

I can be known, by 2 different names
Excluded by neighbours, although I’m the same

Found below the sea, or a small step from the middle
Having a fiddle might help solve this riddle

To have your wits about you is one of my definitions
And navigators love me, for smooth sailing conditions

Of all these things, I do consist
But the truth is, I don’t actually exist

What am I?

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