I meant that they will do poorly in straight seawater. Many will simply not survive. Your'e better off using brackish or even freshwater.
Not true, these are frontal dune plants designed to take on the harsh conditions:
"Only a few plant species can tolerate the
stresses of a dune environment, particularly
frontal dune sites. Foredune plants must be
able to survive being buried by blowing
sand, sand blasting, salt spray, salt water
flooding, drought, heat, and low nutrient
supply (2, 4, 15, 22). Salt spray, by
providing potassium, sodium, calcium, and
magnesium (32), is a major source of plant
nutrients in dune soils. In the absence of
salt-bearing onshore winds, many coastal
dune plants grow poorly or die"
KNO3 definitely provides usable potassium as well as nitrate. There are a couple places you can buy it online - I had a great experience buying from NilocG. Good price and fast delivery.
If you want to try mixing it in solution I highly recommend checking out Rotala Butterfly. They have a calculator that does all the math for you; if you choose the volume of your solution and the volume of your dose, it will tell you how much dry fertilizer to add to the solution AND calculate what the ppm of both K and NO3 will be in your 5 gal tub after dosing.
I was leaning toward GreenLeafAquariums for KNO3