Definition borrowed from
http://www.trimscrew.com/IPE.htm
"IPE (aka. Ironwood) is one of many commercial names used for the the imposing Lapacho group of trees from the various species of Tabebuia.The trees generally grow from 140 to 150 feet, but some can reach heights of 200 feet. Some other common names for the trees from this group include Bethbara and Lapacho, and a host of names used in the countries where the trees grow. The trees are mostly found in Brazil as well as throughout Central and South America and some of the Lesser Antilles.
IPE'S strong, tough resilient properties make it an excellent material and increasingly popular choice for commercial/residential decking and outdoor furniture. It is prized for its stability, durability, strength and natural resistance to decay, wet conditions, and infestation by termites and borers. It is available in long lengths and relatively easy to season. Ipe has a Class A fire rating, the same rating given to concrete and steel."
I wished I used it. The thing with the seasonal lift up docks is that the decking is not really attached to the frame of the dock itself. Small sections of 4 or 5 boards are built on individual frames which are just placed on top of the aluminum dock frame. The problem with the less expensive route of PT in this application is that it tends to warp when there is nothing substantial holding it place, as mine already has after one season.