Abloy padlock, IP68 rated for continuous submersion.
(I have no idea if it's good or not. I just searched for "IP68 padlock."
Case-harded steel, zinc-plated, then polyester coated. If the polyester gets damaged and the zinc plating gets eaten away, it'll rust. I guess they didn't go with stainless because of the strength difference and cost?)
It still amazes me how many things in outdoor applications are just zinc-plated or black oxide-coated plain steel rather than some grade of stainless.
Some gas stations in the area use steel trim around the pump islands. The ones that have painted steel look lousy: Anywhere the paint gets nicked, it starts rusting the steel quickly (I'm in the "Rust Belt" part of the country), and then it spreads and bubbles off more paint.
A few have started using stainless steel, and it looks like it's holding up quite well.
(Stainless still isn't invulnerable. Basic 18-8 or 304, especially with a rough surface texture, will not hold up well when constantly exposed to saltwater. 316 with a smooth finish will do much better, but it costs even more than 304.)
Pay for it now, or pay for it several times later.
I remember hearing in college that 2-3% of US GDP is vaporized every year due to corrosion. Painful.