I had two behave this way... at first, the digits started going haywire after a dive. Some would randomly light up making the display impossible to read after a dive. The first couple of times, it corrected itself after a few hours. Fortunately it didn't affect the ability to download the dive data.
Less than a month after this odd behavior started, we went on a dive and the same thing happened... only when we got home, the computers were in H2O mode as you describe and nothing I did would change that. I even replaced the battery and it turned back on H2O mode. It makes me think you had nothing to do with it by changing the battery since Oceanic's tech support suggested I do that before sending them in for repair.
Since nothing worked... even waiting several days, submerging it in water, etc., I ended up sending them in to Oceanic. I also had the option of taking them back to the LDS, but the turn-around time would have been longer and I would not necessarily had a PP2 as a loaner.
Oceanic ended up replacing both computers. I'm guessing there was a small run with this problem and I got lucky enough that both computers had the same problem... But it was a familiar problem to tech support and I got the new computers back in less than a week. Their policy is they ship back the same way you ship to them.
I didn't try to dive when mine was in the H2O mode, but prior to that it seemed fine, except for not able to see the digits clearly. And never had trouble downloading dive data.
Ken
