Like the name implies, Ironwood is both hard and HEAVY...coming it at around 70 pounds per square foot. It is used on the boardwalks in Atlantic City and Cooney Island. You have to have special tools to work with it do to the hardness of the material.
If you figure a "standard" swim platform is 15 square feet, you are looking at over 1000 pounds of swim platform! If your boat comes in with a dry weight of 3000 pounds, you have 1/3 the weight of your boat on the back end. This will really change how the boat skis/handles/etc.
I recommend sticking with Teak. It's easy to work with, easy to maintain, and still cost effective.