Although I don't have an Outback, I do have an Impreza Wagon that I took through what I'm guessing was 16" of standing water @ around 5mph. It came up to right over my hub which I measure out as 16". The EJ25 in both my vehicle and the Outback is similar and from what I gather the intake is mounted in the passenger fender at around 24". The diffs in Subaru's are not sealed, and along with a lift and other bits planned for this weekend, I will be changing out the diff oil. I didn't plan on crossing through that much water and actually tried bypassing it by taking what I thought was a tame trail made to bypass the water, which ended up being a muddy mess. On the way back I opted to drive on the hardpacked trail with the standing water and drove at what I reckoned to be a safe speed. Knock on wood I have nothing howling and no problems from that trip.
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I'm guessing the water was that deep about half a mile, and I just kept to the right side of the trail to keep my intake high.