Maybe this is because of the endless hours of instruction I have received in how to avoid getting bent. Unfortunately none of the training agencies has thought to put weeing at sea as part of any course. I'm surprised it isn't a PADI speciality. Reading through any of the diving instruction manuals you might be forgiven for thinking that divers don't wee - like the Queen Mum or something. It took me two years even to be able to wee in a wetsuit underwater.
People usually end dives due to air, decompression or cold considerations.
I barely needed to work out my tables or air requirements. I would always end my dives cos I needed the loo. The amount of gas that got wasted doesn't bear thinking about. For people who don't get embarrassed it must seem strange that others put themselves through so much pain. It's not just as simple being told the best way to do it.