The Expansion Kit extends the Ironclads system to 1879, adds ships from major European powers of the same time period, and features some optional rules (e.g., land and land-sea action, designated targets, double/triple gun loads, sharpshooters). New scenarios also, many of them set in the American Civil War, but some in South America or Europe.
Yet another expansion, called Shot and Shell, was later done by 3W. Shot & Shell is a stand-alone game, but it's designed to also expand The Ironclads (even includes a conversion chart).
Note that "Excalibre Games" reprinted both The Ironclads and The Ironclads Expansion Kit. Some say it is of inferior quality to the original Yaquinto version (the ships show side views instead of top views, and the counters are thinner). The box-cover image shown above is the Yaquinto version. If you look at the additional images below, the first one shown is the Excalibre version box, and all but one of the remaining images are of the Excalibre version.
Expands: