Dive shops may try to sell you a new computer. I think most of the dive shops are credible and want you to dive safely. Given the age of your computer it may be best that they recommend a new one, whether it is needed may be another issue. Since, you had the moisture issue inside the computer, it may have - at the least - initiated corrosive properties inside the unit. If the battery does, in fact need replacing, that’s probably the cheap fix. With the age of your computer, maybe it’s better to err in the name of safety. You certainly don’t want this to happen at depth on a dive in the future. It would also be good to check with the manufacturer, Oceanic, to see if they can problem solve your issues. Screen going blank, not good. Maybe it’s happened others in the past, and they may be able to fix it or recommend that you should, in fact, replace the computer. As always, safe diving!