There are no official answers from the adventurer's league at this time. However, I think we can apply some rational thought and come up with a good answer in the meantime.

• All Adventurer's league PCs start out at L1s, Wild Shape is a L2 ability, so I would track what you've seen while adventuring. You might be able to make a case from your background of what you've seen prior to your adventuring career, but it might be a hard sell to an organized play DM.

• I would definitely do this. At the very least for the ones that don't show up in the PHB that you'll encounter. I doubt you'd be called on it, but it wouldn't hurt to have a menu.

• For beasts that are not in the PHB (I'm not sure there are any right now, but just in case), you've got some options. Between the BD&D DM book, and the HotDQ online supplement, you can easily find any beasts that might show up in the current season of organized play that are not in the PHB (and going forward, they plan to release similar supplements for future adventures to keep BD&D the only required materials). Everything in those two online supplements will also be in the Monster Manual that is coming out at the end of September.

Ultimately, I don't think this is something to get too hung up on. I find it improbable that you wouldn't be able to make the case to even an Org Play GM that you hadn't encountered a certain beast before. I'll note that even org play GMs are given this kind of latitude in the instructions to them. However, I also wouldn't make a big stink about it if the DM said "no, you can't be a Stirge, you haven't seen one of those yet" if you don't have it written down that you'd encountered one.

I reached out the D&D adventurer's league on twitter to get something slightly more definitive and here's the response:

wax eagle: quick question. For Org play, does a druid get a starting list of beasts encountered? How do they track them as they go?

D&D adv league: No, the Druid class states what CR the animal can be.

Sorry, I was unclear. Wild Shape specifies "that you've seen before" is that in your adventuring career or can it be before?

That's left up to the players as to what their char has seen.

So the official response is "whatever you think makes sense for your character", that's pretty much in keeping with the spirit of 5e's rulings so far and makes sense. Again they've given 5e org play GMs (and players tbh) wide latitude on stuff like this, so decide something that makes sense with your characters background and story.

The Rules Don't Say

As nvoigt pointed out, the PHB describes chain mail and plate mail as complete items. They are either on, or they're off. There are rules for how long it takes to put them on and take them off, but there is no description for your AC while wearing just the padding.

Furthermore, the padding under plate and chainmail isn't padded armor. Padded armor is a different thing. When you buy plate mail you don't get plate mail armor that has padded armor inside, you get plate mail armor that has padding inside.

You Could Try It

Since there are no rules that describe this situation, you are down to the most general rule of all. At the beginning of the Players Handbook, it says:

1.The DM describes the environment. ... 2.The players describe what they want to do. ... 3.The DM narrates the results of the adventurers' actions. (PHB p.6, "How to Play")

So, your character could just try it and see what happens. "Try it" could mean a lot of things. "My character takes off all his armor but the padding. What's his AC?". "I'm looking at the swamp we're about to cross. I'm an experienced fighter, I've been wearing armor for years, I've been trained by experts. I know if I fall in wearing full plate, I'll probably drown before I can get out of it. What does my training tell me about whether I can wear just the padding in order to mitigate the drowning risk but still retain some protection?" Or perhaps your armor wearer might consult armor experts.

You've Solved My Drowning-in-Armor Puzzle!

As a GM, I view the water-armor situation as a challenge or problem for the PCs. Which do you do, take off the armor and carry it thereby lowering your AC, or wear it and risk drowning?

Only it isn't an either-or, which is the beauty of RPGs. There are other solutions to the problem. Perhaps the water is such and the PC's strength is such that the PC can just walk out, perhaps with a strength or constitution check. Perhaps the PCs have access to magic that allows them to breathe water. Maybe they can bypass the swamp. The players might (and hopefully will!) have innovative solutions I haven't thought of.

If the PC said, "I take off most of my armor, and wear only the padding", or "only the helmet, gauntlets, greaves and leg guards" and subsequently fell into the water I think the time to remove the armor and thus the risk of drowning would be less, while the chances of swimming in part of the armor would be more. And if the PC were attacked by Rodents of Unusual Size while crossing the swamp, I'd definitely have to allow that the PC's base AC in padding or other bits and pieces was greater than without armor. Wearing only part of the armor might be a reasonable solution to the but-I-might-drown problem. Although wearing only the padding might be really bad for it, given briars and such.

Given that I don't want to play "Armor & Anoxia" I'd be reluctant to come up with a codified houserule that says you can wear half the armor for half the AC and half the risk of drowning, but perhaps that would be the solution. I'd definitely want to keep things liquid, and not weigh the game down with heavy armor questions, but I'd definitely want to pad it enough to keep things moving forward, while at the same time rewarding innovative role-play and problem-solving.