Do NOT drink a shitload of water. One time I drank an entire bottle shortly before we began our descent. The plane would continue to circle for almost an hour and I was shaking like a madman since I really had to take a piss. I couldn't go to the toilet as it was slightly turbulent and everyone was told to take their seats as we would land soon.
After we finally touched down, I was just about to piss myself, but I kept my patience. Then as soon as the plane halted and the seatbelt sign was switched off, everyone jumped out of their seats to get their luggage and I realised I had to wrestle through all of them while at the same time holding it in, to get to the bathroom. I was slowly pushing myself forward, quietly excusing myself and mentioning I had to use the bathroom. Luckily, a brawny man heard me and started screaming "he needs to take a piss!! move to the side!!!" or something along the lines of that. Everyone moved back in front of their seats and I had a direct pathway to the toilet.
As soon as I arrived there, I noticed the door was locked and it said occupied.. I was just about to release it, when a stewardess unlocked it (apparently they lock it under landing, there's a hidden lock behind the name-plate which allows you to lock and unlock it from the outside) and I jumped in, pulled my pants down and had the greatest piss in my life.
Anyways, like said earlier, it's navigating the airport that's the worst part. I flew alone from South-East Asia to Northern Europe when I was 13, with only a single exchange flight at Schiphol Airport, which is really damn organized and simple, so it went smoothly.