The answer is, if you submerge your diffs enough, and you aren't extremely anal, then you won't automatically change your diff fluid but rather inspect it, and change if/when it gets milky. If you are anal (conscientious might be a better word here

) then you change it regardless, because gears are expensive, oil isn't (well, maybe Amsoil synthetic if you change it enough), and you just never know.
All diffs have 'breather' nipples to equalize pressure changes (due to thermal expansion) and avoid blowing seals/gaskets. A submerged diff (the theory goes) gets cool quickly, sucking in water through the breather. By raising the breather above the waterline, you usually avoid this problem.
Unfortunately, most of us are never sure our extended breathers are air-tight, or we do them half-assed and they fall off, or kink and cause seals to blow oil through, etc. I wouldn't do them unless you ford water regularly, AND are willing to inspect them for proper operation, AND install or have them installed properly. Just drive it wet for the day, and change the diff fluid later, if you get into water. (count me a non-anal guy

)
For the opposite of extremely anal, how about this: a vehicle driven with no front diff cover (for the Dana-style diffs). The alternative definition of 'open diff'?

The gears were exposed to the elements! Apparently the guy dented/ripped the cover on rocks, and went the rest of the day with the cover off.
Kenton