Those are great temps for 250ft
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It's the decompression obligation. Basically the "ceiling" you have to stay under.
You are correct
Wow, over two hours for 25 minutes of searching time - and so long hanging at 20+ feet in murky water off gassing, over an hour of ascent and deco time.
The graphs are from a liquivision X1
The smooth line is temperature (right side scale)
idive300 from H2Oaudio is your friend.