Last year they were giving away free bottles of one of the vitamin water type drinks before you walked across the boardwalk to the main entrance. I hadn't finished it prior to walking in and thought they might make me throw it out (I think they did make me discard a plastic bottle the previous year prior to entering), but they let me walk right thru this time. I think that officially you're not supposed to bring any bottles in (definitely not glass), but it's probably hit or miss depending on who the usher is. The main thing is they probably want to make sure that no glass and no booze come in.
So, if you like to drink a lot of water, and you will if it's a steamy day, worst case scenario is that you'll have to buy a bottle of Evian for 5 or 6 bucks when you get in, and then you can just refill it at the water fountains the rest of the day. I think the water fountains are relatively decent and reasonably cold, but you Brits don't like your water cold anyway, right?
If I'm there one of the days you're around, I'll buy you a $10 beer, Adam.