They are fairly rare, often disguised as smaller gas giants. The handful (less than 10) I've found have all been in the 20-40 Earth Masses range and of the same color, like the one in your screenshot.
You will appreciate the value of Earthlikes even if you aren't exploring for the profit lol.
I'm at about 12,500 systems and I've found on average, 1 per 200. Certain areas of the galaxy (clusters of type F,G , K stars, neutron fields) have a higher chance, but I don't seek them out so I don't find them as often.
They're like Helium Giants inasmuch as that the cirumstances of their formation are very rare and they tend to come in clumps - if you're lucky enough to find one in a system it's worth checking the other giants to see if there are more.
After 1.3 they gave them the same holo icon too.
Basically, if they sound like water streams and are relatively small for a gas giant, they can be Water Giants.
I have only found one or two of these in my career as an explorer as well... It seems they are all murky brown-grey. It would be cool to find a clear blue one..