First, as pointed out once above, cycling uses hybrid bearings when they go ceramic -- the races are steel, the balls are ceramic. So lubrication is definitely required and is defined in the spec of most ceramic products.
Second, it's more often the races than the bearings that actually go bad. Even with steel bearings, you had to go back thirty years to find bearings of such poor metallurgy that they'd flatten or crack. If you examine a bad bearing, whether headset or bottom bracket or whatever, it's usually the races that have gone bad and have pits (typically if there's limited rotation, as in a headset) or rough grooves (for the other applications).
Third, seals are basically the same for most components, whether bearings are ceramic or steel.
Next, ceramic bearings today are extremely durable (early versions weren't, and tended to crack too easily). They can take everything you can throw at them including water, grit, and so on. However, you still have the races to deal with, so ..........
Also, one of the benefits of ceramics is that the bearings can be made much more perfectly round (or any other shape you need) compared to steel. Go to YouTube and watch some videos of ceramic bearings being made -- they are spun on air jets as they're being made, so they attain symmetry beyond what can be done with steel. That's really why you get those slight wattage savings when going with ceramic bearings. (It's also why you benefit from going to oversize jockey wheels with oversize bearings, because the bigger the bearings, the less sensitive you are to minute irregularities in the bearings or the races.)
You asked if ceramic bearings were better in rainy conditions. Because it's only the bearings that are being replaced for steel, it's not a lot of weight savings. The ceramic bearings are going to be, if anything, a fair bit more durable than steel bearings, but it's likely the races that will go first anyway, so that doesn't amount to much. Both types of bearings need lubrication and typically have similar sealing. The ceramic bearings will save you a couple watts, but in the rain you'll be losing that elsewhere. In short, I'm not sure I see the benefit you're looking for. Especially when you have to pay a good bit more for it. If those two or three watts are really the difference for you between winning and losing, then yes, go for it. Your pro team will replace the bearings as quickly as is needed and will pick up the bill. For most people, ceramic bearings won't hurt, but they don't do much. Even less so in the rain.
Thanks for the clarification!
That means that for rainy weather either regular bearings or ceramic bearings BUT then they have to have a proper seal (as I understand that some manufacturers removed the seal in order to achieve less friction), right?
But how do I know if a wheelset have a proper seal or not (if they have ceramic bearings)?
for example the Roval CLX 50 Disc ("Roval AFD1/ AFD2 hubs with CeramicSpeed bearings")