Im trying to source a product that can be painted onto steel 'rails' on a ship hoist to prevent corrosion while the rails are submerged in ocean water (for approx 3hrs straight).

Currently they use a hardened bitumen coating which I've been told forms cracks after three or four dips (I'd imagine the steel gets quite hot through the day and cools quickly when it hits the water). As you can imagine its not a very cost effective exercise and are looking for alternatives.
They've mentioned a product called 'texaco blackjack' (which is out of production) which was a semi fluid bitumen based protective marine grade coating which apparently lasted several times longer.
So far we are trying out a lanolin grease (which in spray form has failed) and an open gear lubricant. Any other suggestions?

You can use a rustpreventive coating. These will not crack, and you get them in different grades. Choose a heavy one.
Try one of these: Esso Rustban, Sun Oil Anti-Rust BSH, Shell Ensis.

THis will have to be a system that is Akaline or based as this will "soak up" the free electrons supplied by the polar sea water that catayse the corrosion.
Calcium sulfonated waxes and or greases would be a good starting point, car under body system may be of use