I have never spec'd an aluminum ladder for use under water. For potable water tanks, I have always seen like materials for the ladder and the tank (ie, steel and steel), both of which are coated with epoxy based paint. I have also seen aluminum ladders for sewer pump stations...all of which had to be removed due to significant corrosion (this may be the result of chemical corrosion as oppossed to electro-chemical).
Is this tank steel, and if so is it coated? I would consult with the ladder manufacturer on this one. Given that Aluminum is more anodic than steel, the aluminum runs a good risk of corroding. There are several other factors to consider as well, like pH of the water, what other materials are present in the water (maybe a sacrificial anode?). Also don't forget the little things such as the bolts...steel in contact with aluminum under water.
Given the different variables that could be encountered, it would be well worth it to play it safe and use like materials. Otherwise, I would consult a qualified Corrosion or Metallurgical Engineer.