hydration options on a P2

What bars do you have?  You can zip tie a cage up there and use a regular bottle.  You might have to decide how you want to place your bike computer but that's about it.  It's the easiest and cheapest thing to try.

Edit: also if you don't want to get your bars all dinged up, you can put some electrical tape over the areas where the bottle and cage touch.

Here's my setup.

+1 on a zip-tied cage between the aero bars

I also have a P2 and I use the Xlab Delta 300 in the rear. LOVE IT! Pricey but it does come with the Gorilla cage...

http://www.amazon.com/XLAB-Delta-300-Single-Gorilla/dp/B00AU8BZE2

P2 and Adamo Road here, too. I have an X-Lab cage between the aerobars (not realizing at the time that zip ties would probably do just as well). I've been happy with that solution.

I also have a P2 with adamo race saddle.  One bottle on the down tube, and a cheap metal cage zip tied between the bars.

same here. cheap and useful

The setup between the bars is what I have, but it all depends on how comfortable you feel taking out the bottle drinking and returning it while aero. If you think that will be too complicated you could try something like this:

http://www.discounttrisupply.com/product/Speedfill_A2_Aero_TT_Tri_Bike_Hydration_System.html?Search=a2

or like the torhans

If you have a bike computer be aware of how you will set that up, too. I put a PD aerodrink on my P2 last year and I'm not happy with how it interferes with being able to read  the Garmin 800. It's mounted too far back to be able to read properly in aero.

I really like the new PD bottle, though. Speedfil standard and an Xlab wing might work well, but with two 22oz bottles that's over 5 pounds of weight.

I can squeeze an aerodrink between my aerobars on the front, but your bars may be set a little narrower.  On the back, I have a Minoura mount with two bottle cages (I have used it with both an Adamo and a Cobb) that has worked out well.  The Minoura mount is very cheap, too, as far as hydration options go...

http://www.newegg.com/Product/Product.aspx?Item=9SIA1G80DV4978&nm_mc=KNC-GoogleMKP&cm_mmc=KNC-GoogleMKP-_-pla-_-Cycling+Accessories-_-9SIA1G80DV4978

This is what I have along with a zip tied bottle on my aerobars.  Works fine.

yep, another with a zip tied metal cage between my bars.

I keep the nutrition in the down tube cage, and water for exchanges up front, often times the bottles at exchanges are smaller, so just bend the cheap metal smaller to hold the water bottles at exchanges.