In One Piece Episode 782 and most of the appearance of Kuzan, the screen shows that Aokiji is riding his bicycle through the sea on the ice which was transformed from seawater. How can he turn seawater into ice even though devil fruits powers are neutralized with seawater? He should be able to ice the ordinary water, not sea. Any thoughts with me???
There is no stated reason why seawater could not be frozen.
Logia devil fruits's powers are not stopped by salty water. The Mera Mera fruit can create flames to burn salt water, for example. As long as the fruit user's body is not submerged, his powers are not affected.
Another explanation (not my favorite, though) is that Aokiji just created ice, just like other users can create elements (like smoke and fire). If he did create ice, then he simply layered his ice over the ocean, which again has no contradiction with the effect that seawater has on fruit users.
It is stated by the creator Oda Eiichiro in an SBS that only until one is submerged knee level or more in water does the weakness start to take effect.
The weakness can be caused by all types of water not just seawater. Also it happens in bodies of water and not in, for example, rain.(Stated in SBS vol. 41)