Can't speak with authority on the aluminum corrosion issue, but my gut tells me this isn't good.
You'll need to guarantee a high enough flow rate to ensure that the exiting water temperature is low enough to not damage your piping (most likely PVC, right).
That will also guarantee that your exchanger metal is below the condensing temperature, and the flue gases will almost certainly eat your tubing from the outside in, in short order, unless you can make this out of something like titanium, or short of that, 316Ti Stainless.
What about doing this with a closed loop, so you don't pump the pool water through your core?
Say something like this:
Take an old boiler, and raise it up so you can light your wood fire beneath it. Fill with clean water, and install an overpressure safety valve, and expansion tank.
You can use cast iron here, and even a cast iron pump (say a small Taco 007), to pump water through a loop.
Loop around 15 feet of PEX in your pool, and pump through this loop. Set up the aquastat to only pump when the core temperature is above 140F or so (to stay out of condensing mode). You just need to be sure to choose a pump large enough to make sure that the temperature can be kept below 180F (for the sake of the PEX.
As for having plenty of wood, here's something to consider:
18000 gallons of water weighs 144000 lbs. It takes 1 BTU to raise 1 pound of water 1 degree F.
There's a bunch of data on how much heat you can get from wood, and I found this on google really quickly:http://www.daviddarling.info/encyclopedia/W/AE_wood_heat_value_BTU.html
So, using an average for Oak, you should ideally need around 33lbs of wood to heat your pool 1 degree. Kinda reasonable I guess.