I wish all ND filters had dual markings. Some say things like "ND3" or "ND10", others say "ND 8x" or "ND 1000x". In any case, realize that what the former method aims to tell you is that the required exposure time goes up by "2 to the power of x". Examples: an ND3 extends the exposure time by 2 to the power of 3 = 8 times, an ND10 by 2 to the power of 10, which is actually 1024 but everyone refers to it as 1000 times. These filters cannot be made that accurately anyway, so that difference does not matter.
Stacking multiple filters using the Cokin P system may seem like a good idea, except that Cokin's filters have a reputation of causing pretty bad color casts, which would be further amplified if you used multiple ones. This would be less of an issue if you used the more expensive Lee filters.
But back to the original question: if you want that silky water look water in your ocean pics, you'll need to expose for several seconds, even tens of seconds. With your camera set to low ISO and an aperture of, say, F13 or F16, in bright daylight the resulting shutter speeds will still be pretty fast without that filter. Multiply those by 64, which is what a ND 6x gives you, and you are still not anywhere near those longer exposure times you want. Because of that, I think an ND10 / 1000x is about the lowest you'll want; you may even find yourself longing for a filter combo, for instance putting an 8x on top of a 1000x, to give you plenty of flexibility and avoid having to close the shutter further, thereby reducing the image quality.
