Contradiction with Genesis 1:9
This verse describes the gathering of waters in one place, whereas Joshua 18:19 indicates a specific boundary along the waters.
Genesis 1:9: And God said, Let the waters under the heaven be gathered together unto one place, and let the dry [land] appear: and it was so.
Contradiction with Isaiah 43:16
Isaiah speaks of God making a path in the mighty waters, contradistinguishing the specific boundary mentioned in Joshua.
Isaiah 43:16: Thus saith the LORD, which maketh a way in the sea, and a path in the mighty waters;
Contradiction with Revelation 21:1
Revelation refers to the sea being no more, which contrasts the emphasis on water boundaries in Joshua 18:19.
Revelation 21:1: And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea.
Contradiction with Exodus 14:21
This verse describes the waters parting, a direct action by God, unlike the fixed boundary described in Joshua 18:19.
Exodus 14:21: And Moses stretched out his hand over the sea; and the LORD caused the sea to go [back] by a strong east wind all that night, and made the sea dry [land], and the waters were divided.