I had an o-ring rupture on my transmitter on a dive recently, during our surface interval. No big deal, just swapped out the o-ring. As far as the battery, $23 for 2 spares and 2 o-rings isn’t a huge amount, especially in the scuba world. Just like all of your other equipment, check it before you go on your trip. I have 40 dives on my current batteries (2 years old) and they still show a good charge. I bought my son the same computer and I can keep tabs on his air also when we dive.