I haven't used krylon submerged, but I had heard that you were supposed to let it cure for something like 7 days before submerging it, so that might make a difference.
Supposedly it chemically bonds to the plastic, so if that's true, then as long as the piece of plastic is clean, I don't think you would need to sand it, since that's to help with a mechanical bond.
If you can find a plastic supplier near you, you should be able to get black colored acrylic.
Otherwise, I'd look for other sources of flat, black plastic pieces. Try wandering around the hardware store, and look at cheap trashcans, washing machine/waterheater/shower trays, cement mixing tubs, etc...