Yes, muriatic acid is denser (heavier) than water. With some mixing/circulation going it will mix with the water quickly enough that that doesn't really matter. But in still water it will tend to sink to the bottom and puddle there for a while before it finishes mixing.
Good question. I didn't know HOW much heavier so I googled it and found something interesting......It's apparently an in-stock item at many Tractor Supply Stores....$4.98/gal...I've paid 7.90 @ Lowe's if I remember correctly.