During Murphy's transformation into Robocop in the original movie his organic form is almost totally replaced by machine, as overheard in a discussion between Bob Morton and one of the scientists:
Tyler: [while creating RoboCop] We were able to save the left arm.
Bob Morton: What? I thought we agreed on total body prosthesis. Now,
lose the arm, okay?
It's never made clear exactly what constitutes "total body prosthesis", however we are told that Murphy's remaining organic systems require nutrition (by ingesting a nutrient paste). Whether they also require oxygen in the traditional manner (breathing) is not actually clear, Robocop doesn't appear to have blood or veins and arteries in the normal sense and so would be unlikely to need lungs to breathe with. His only external organic feature, his face, is flesh overlaid onto a metal base, apparently for cosmetic purposes. So it's possible that his organic parts are sustained without the need for breathing air and hence he could survive underwater for some undetermined amount of time.
That said, a (probably non-canon) Japanese "Making of Robocop" book published in the 1980s shows Robocop having an "emergency air supply capsule" in his left leg which implies he does need air after all. In that case he would be able to avoid drowning by consuming this emergency air supply, but could theoretically drown if it ran out while submerged.
EDIT: included a copy of the picture from the Japanese book: