Skidding on Rear Tire in wet weather

This is to be expected.

Bicycles have most of their weight on rear wheel on flat land, so some amount of braking using the rear brake is possible. Braking tends to skew the weight distribution towards front, so a rear brake is never as powerful as a front brake. Thus, most experienced cyclists, if riding on pavement, only use ever the front brake and never use the rear brake. An exception to this is maybe hard cornering -- if you need to brake during hard cornering, you have only very little braking power available since hard braking would lose traction and make you crash, so it might be beneficial to equally apply very lightly both the front and the rear brake during hard cornering.

When going downhill, the weight distribution is even more skewed towards the front. Even without braking, on a downhill you might have more weight on the front than on the rear. Brake a little and the weight distribution becomes even more front-heavy.

So, the rear tire has very little weight on it during braking. In wet conditions, the coefficient of friction is worse so that makes it even more problematic to use the rear brake on downhills.

Learn how to use the front brake. Note that unlike rear where skid is recoverable, when using the front brake you can raise your rear wheel (recoverable) or skid the front wheel (not recoverable). Front skidding can happen when there's sand on the road (acting like little ball bearing balls), or if riding on the wet over manhole covers or painted lane markings or other road markings. Also do note that the tar patches used to patch cracked pavement are very slippery in the wet.

Basically, sand is slippery and in fact, everything except normal pavement is slippery when wet.

Even on wet downhills, I would not touch the rear brake at all, unless the front brake fails in which case you have no other option than to skid the rear wheel. Every lane/road marking, tar patch, sandy portion and manhole cover should be a location where you temporarily stop braking.

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And an addendum, some time ago it used to be fashionable to fit road bikes with colored tires. Those tires can't have carbon black in the compound, because that would always make the tire black. So they had to use silica instead of carbon black to make coloring the tire possible. Not all of these silica tires were colored, in fact some were black (although may be a slightly different shade of black than that in carbon black tires). Silica has very poor traction when wet, so if you have a silica tire you might want to replace it with carbon black tire. Also tires age so ten-year old tires should be discarded even if they have enough tread depth available.