Yellow polypropaline rope will hold up the best in sea water. Down side is that it floats and it is suseptable to sun light.
You next best rope is Nylon. It loses strenght when wet but will hold up in the sun.
For simple, low load and temporary lashings, use manila because it is cheap, cust eaisly and keeps it strength when wet. It will degrade faster than any man made rope though.