I'll help you out. In the battle against Rahvin and Sevanna's boyfriend's armies, Rand attacks forces up to 5 miles away. He then also says that this is effectively the maximum distance he can reach. This was with the little man angreal, so you can maybe half that distance to be conservative, for when he channels unaided. Although it is not certain that he would have to reduce his distance without the angreal - he may just not be able to make weaves as powerful, but at the same distance.

Either way, we are talking a distance of miles here.

As for not knowing the sea floor - he has his new trick from tGS, described by Nynaeve.

But even better, he can simply make a gateway from in front of his ship to the sea floor - facing AWAY from him. The result would be a jet of water of unimaginable strength blasting out of the opening away from the ship. Then all he needs to do is spin a second gateway, facing the first one, 1 milimetre away from it. This second gateway opens into space.

The result is the massive jet of water from the first gateway blasting straight through the second one and emptying into space.

Now he just inverts and hides these weaves with Illusion, and Bob's your uncle.

Hell, he doesn't even have to use a ship. He can sit in a cave in the Blight somewhere and complete the whole operation in minutes. In that case he just needs to make the gateway to space first, and the one opening to the sea floor, second. And even better, he should make the opening into space maybe a foot larger than the one into the sea, to make sure the entire water stream from the smaller gateway fits into it without any hassle.

And lastly, he should encapsulate this little contraption with a sphere of Air that is sealed off, unless he wants the earth's atmosphere to also drain away into space. Heck, come to think of it, maybe he actually WANTS to do that too.