Major D - are those drills the "kick your legs really fast while gliding for a few seconds before launching into stroke?" I had a session with Julian last year which I thought was excellent and he prescribed that for my sinky legs, which improved immediately. I then didn't do the drills ever again and hey, I'm still crap.
Shitty brown water.......nah it's just full of aviation fuel from the London City airport, they used to pump water straight from the dock to use when they practiced crash drills, till they realised the flames weren't so much going out but fueling the fire and making it bigger.
Sounds like an urban myth to me - given that fuel is less dense than water, the fuel would float at the top and you'd be swimming in it and drinking it. To my knowledge, no-one has got aviation fuel poisoning after doing the London Triathlon
London was my first tri back in 2010, I only just broke an hour for the swim (59:59 officially). I even had my own safety canoeist shepherding my round, we had plenty of time for a long chat, Dave was his name. As others have said, it's not that bad - and if you're really slow you might even get the personal service that I got so no need to worry - enjoy it!