POR-15 is a chemical bond to the exposed metal, best I can tell.
It is a mechanical bond. When used on new smooth metal it will come off in large sections later. I have seen this happen a few times on the fender arches on new body panels on cars.
It is simply a moisture cure paint so it mechanically bonds to the rough rusty surface and uses the moisture in the metal and in the air to harden. It attempts to seal any future moisture out.
Por15 is not the only paint like it. There are other brands that are the same thing.
There are other similar paints that would be a better option for water immersion
If I were you, I would look into using Rust Bullet paint for water immersion.
They specify marine use (in addition to other applications) and have test results/data, unlike por15 and similar paints.
Rust bullet, depending on the type you use, can be much thicker than normal paint.
In my experience It also bonds better to new metal. It also doesn't change color when exposed to UV.
If it is good enough for tugboats and battleship "Texas" then it should be ok on your keel. Hehe.
https://www.rustbullet.com/marine-applications