I think it goes without saying that softer paints need softer bristles. In my personal experience, the best brush for water colors looks a lot like a calligraphy brush (just a bit smaller). You should look for thick brush heads, long bristles and a pointy tip.
Thick brush heads means that there are a lot of bristles in it. More bristles means more tiny spaces between them where water can be stored.
Long bristles can also store slightly more water than short ones. They also tend to move more fluidly and have a softer stroke. It's hard to explain, but I had better experiences with long bristled brushed for soft paints like water color.
A pointy tip means that the bristles in the center are even longer than at the perimeter. Those longer center bristles are the ones you want to actually paint with. The ones around the perimeter are mainly there to store water or ink and to keep the center bristles in shape.
Here you see that only a small number of bristles actually touch the paper and write. Yet traditional calligraphy brushes have these thick brush heads to retain ink and their shape.
The same can be applied to synthetic watercolor brushes. On the largest brush you can clearly see the shorter bristles surrounding the longer ones at the center.
As for labels, most manufacturers I know simply label them as "watercolor" or "aquarelle" brushes. I would buy one medium size and one big size and don't bother with small sizes too much. Since they should all end in a fine point, a medium brush should be just as precise as a small one and the small one is just worse at retaining water.
In the picture above, I'd chose number 8 for medium size and 14 or 16 for big size. Number 6 has visibly shorter bristles than 8 and even number 0 and 2 don't have a finer tip than 8.