Can I say "regarde" instead of "voit" (even regarde meaning "watches")?
"Elle regarde la mer" would be "she looks at the sea".
"Elle voit la mer" is "she sees the sea".
The two sentences don't mean the same thing.
Is there any difference in pronunciation between "Mer" and "Mère"?
A good way to remember the gender of la mer is to think of "au bord de la mer" !
I remember it because of the famous piece by Claude Debussy, La Mer.
George, I enjoy the music references. Here is a slightly newer "La Mer", George Trenet, later covered by Bobby Darin: https://youtu.be/PXQh9jTwwoA