This is mostly for Rainer... the urge to pee is not just a head issue. It is a physiological chain reaction that causes the bladder to get full when diving and swimming. This will NOT happen on a car trip because it is a function of being in water.
The medical term is "immersion diuresis" and essentially what happens is that when we are in water that is colder than the surrounding air, the blood vessels in the extremities narrow. The extra blood goes to the core organs, which then triggers a response by the brain to stop the production of "anti-diuretic hormone" (in other words, a hormone telling your kidneys not to produce piss) because the brain figures with all that fluid in the organs that there is an excess that needs to be relieved. This makes the kidneys immediately begin producing copious amounts of urine. Urine flow is estimated to increase 300% to 400%. I guess you can get away with not needing to pee in your wetsuit if your dives are not very long or if the water is quite warm, or if you have an unusually voluminous bladder (some people seem to be camels), or if you don't hydrate sufficiently while diving.
Of course, once you get back on the boat, the situation will reverse itself, and the brain will realize that it needs to put more water into the blood to dilute it. So this water will come from your flesh, not from your bladder. BTW, this makes it especially important to drink lots of water when diving--it's not just the dryness of the compressed air, or the exercise, or the sunshine--we also need to replace the body fluids that went into making pee during the dive.
Personally, I find it very difficult to pee in my wetsuit. I have to be stock still and concentrate (and even push) to make it happen, but I will do this if I have an uncomfortably full bladder. I will also hang from the ladder of a small dive boat after a dive to pee if the boat doesn't have a head.
