Well, latitude and altitude are the main ways to designate zones although, as you mentioned, proximity to large masses of water change things as well. I'm at a similar latitude to southern Cuba and Río de Janeiro and the types of plants that will grow in both locations are similar but the important difference would be rainfall patterns. We can fairly easily compensate for differences in rainfall in our home gardens so I think that is a less important criteria for home gardeners but more important for farmers.
Most would define equatorial as within around 10° of the equator. Most of those areas are warmer with rainfall more consistent at all times of the year than the areas from 10° to 20° north or south. None of these areas are defined by cold but by rainfall and warmth. Personally, I'd be happy to just know what plants need cold to do well, which need low or high humidity and which need a dry and wet season or all year rainfall.
