1. How difficult is it to get a lounge chair? A locker? What is the locker fee and do you need exact change? Should we plan to be there at the opening of the day?
You should have no problem with the chair if you are there early. Any later and you might need to look in the areas of the park that are tucked away. I have seldom seen all the chairs "throughout" the park taken ... it's just that the ones near the water and in the best areas are taken. To some people, this means there are no chairs. We have always arrived upon opening at the park, gotten a locker (which you pay for at a "shack" and get the key) for our valuables (things we cannot afford to lose) and then stake out our chairs. I actually prefer them a bit in the back because then you aren't as crowded. Another advantage of getting there early is that you can get in the major rides/slides before the place gets crowded.
2. Is it helpful to have your own beach towel there to mark a chair? Are these taken chairs respected as such? Is it safe to leave a bag unattended while in the water? (Locking up wallet, cell phone, ipod in the locker, of course!)
We have never had a problem with leaving our stuff. We do bring our own beach towels, but never the really nice Disney towels that we own. Ours are more generic that we bring. I do try to tuck things away so nothing that might be tempting is showing. Anything I cannot afford to lose is put in the locker.
3. Should we bring water shoes, or do they get in the way (stick) on the slides?
I've never worn water shoes; however, you will see enough people with them so you won't feel odd if you do choose to wear them.
4. Will they accept resort ID and use CS/snack credits from our DP?
Yes, it is my understanding that the main food vendors accept the card. Not sure about the little shacks though.
I prefer TL over BB; however, that is a personal preference. I'd recommend that you try to arrive at opening and quickly move along into the park to get your chairs ... then go back and get your locker and lock up the items. At that point, I would start to go on the popular slides, etc ... if the day is hot and it is crowded, lines can build later in the day.
Another tip regarding the "lazy river" and tubes. Do not join a large group of people waiting for tubes. Instead, walk toward the back entrances of the river (example for TL: hardly anybody waiting at the Shark Reef entrance) - "normally" you will find tubes and entrances with very few people waiting. Don't be afraid to get wet and walk in to retrieve a free-floating tube.
Have lots of fun. Remember you can take a cooler in with you. They may check for alcohol and glass, which are not allowed to be brought in.
Have fun!!!