As the title suggests you can not place sea plants such as sea pickles in non water source blocks.
Step to reproduce
1. Create a tank that is at least 2 blocks tall and 3 blocks wide
2. Fil the top block of tank with water
3. Try and place a sea plant in the bottom block of the tank.
You will see that the plant texture eithers bugs with coral or with other plants like sea grass it breaks. Unlike the java edition where you can place theese plants in non water source blocks.