Here is what I had learned once upon a time...
You should avoid peeing in your wetsuit because the pores in your body get deceived. When the warmer liquid hits the pores, they open up thinking the ambient temperature is available to warm you up. So you feel warm for a few moments, but as the water around the warmer liquid (and around your body) cools this warm liquid down precipitously, it drops the temperature faster than your pores can react and you will feel colder very quickly as the cold warm overwhelms the opened pores. That is why you feel warm initially, but over a short period of time you actually start to cool down quicker..
That is not to say I will not pee in mine, but I do avoid it if possible. I try to go before hitting the water, and if I find the need to go, I will do so.. I will try to hold it if I think the dive is ending soon.
I am looking at a drysuit soon though and will probably pass on the p-valve, but am looking at the relief zipper for those pre-dive and surface interval trips.
Although peeing may indicate well-hydrated, it only is an indicator if the pee is near clear. Pee is your body's waste. A heavy color is indicative of unused products leaving your bosy other than just water....
But then all that training and education could be outdated by now....