1.What is the seawater in relation to the cell of water hyacinth? isotonic, hypertonic or hypotonic?

Question

2. Will the water hyacinth survive in seawater? if yes, why? if no, why not?

Answer 1

Seawater is hypertonic because of the increased amount of electrolytes.  

As far as the water hyacinth, it is a plant and plant cells have cell walls which reinforce the cells and allows the cell membrane to pull away from the cell wall and form a sandspur like form. The condition is called plasmolysis.  

Probably not good for long term but would allow the plant to survive hypersaline environments much longer than an animal cell.  In Florida you do see water hycinths in brackish parts of rivers. But not near the ocean so there is some limit.    

Answer 2

Water  hyacinths are fresh water plants and so sea water will be hypertonic to the plant.

They would not survive in sea water since they would lose water by osmosis.