I bought a pack of aponogeton from walmart a few weeks ago. Only 1 of the 4 bulbs grew, but boy did it grow! It was just a few pathetic leaves about 3 inches tall when I left for vacation, but when I came back just a week later it took over. :) I'm kind of glad only 1 of them grew, I don't know what I'd do if I had 2 or 3 of this plant in my 5 gallon.
As far as the sinking, all of my bulbs had sunk right away, but I'd assume if it had been dry for a long time it would take a while to soak up enough water to sink....but then I'm the furthest thing from an expert.