This page has seen huge improvements in the past couple of years. Here is a little something I wrote about the hike:
A major landmark is a large gorge created during the earthquake of 1541 which opened the lake at top of the crater, (hence the name Volcán de Agua) and destroyed what was then the capital of Guatemala, Ciudad Viejo. Agua’s summit now boasts a massive crater including a soccer field and a shelter. No association has yet taken charge of Agua, and it is therefore recommended to ask about safety before climbing, especially if without a guide. Several manned huts exist on the volcano’s summit, and are a decent option for a potential site for emergency shelter.