I don't know if this is the case for all hydrophobic coatings, but I was looking to use them on some stainless canals and pits when I worked at Aramark. I got in touch with a company who sold a product called Ultra Ever Dry. They were the industry leader in industrial hydrophobic coatings at the time. Upon speaking to them, they informed me that if the product is used on a substrate that is submerged in water for a significant amount of time, it actually loses it hydrophobic properties. They told me all coatings were like this. They could not explain to me why because of "trade secrets," but said I would never find a product on the market that would last if it was continually submerged in water. Personally, I am thinking because nothing is truly sealed and everything is porous, then if it's constantly submerged, the water effectively works its way into the pores and under the coating and will eventually lift it. Just my thoughts.
While we are on the subject, I use Scorpion CMX Ion on my gel-coated trailers at work. It has stood up to the constant bombardment of a 3K PSI pressure washer. I'm on the two year point on one of our rentals and it's still holding up to bi-weekly washes on straight gel coat. I also use it on the thin aluminum diamond plate on the trailer steps and rock guards and it works fantastic! The front of the trailers take a ton of abuse from road debris, but the Ion handles it like a champ.