When you Push towards the Ocean with "Sweep Into the Sea," the land you Push to must be closer to the nearest Ocean than the land they're currently in. So, e.g., a land 2 away from its nearest Ocean could only Push to lands 1 away from that Ocean (Coastal to it). As with all ambiguous cases, if multiple Oceans are closest, you decide which one to consider "nearest".
Similarly if you move a piece Inland, you move it further away from the Oceans collectively: each land it moves through must be further from the nearest Ocean than the land it came from. The piece can't move e.g., from a Coastal land on one board to a Coastal land on another board; even thought it's increasing its Range from the Ocean it was originally closest to (from Range 1 to Range 2), it's still just as close to an Ocean (Range 1).