akvo

This is where most learners first meet the accusative of direction, and it is worth slowing down for. En la akvo means in the water, already there; en la akvon means into the water, arriving. The -n is doing the work that English does with two different prepositions, and the same rule applies to every preposition of place. The root also gives akva for watery, senakva for dry in the waterless sense, and akvumi for watering plants. Note that akvo is a mass noun, so quantities take da: glaso da akvo.