A cylinder is filled with 120L of air at 101 kPa and 20*C. How long could the cylinder be used "just underwater" on earth assuming 2L is used every 5 seconds.
I don't understand what the pressure and temperature would be "just underwater." Is it assumed that it is SATP?
Yes, assume 1 atm and around 0 to 20 C
the water would be frozen at 0 (unless salt :) and is probably not much more than 20
that range is small compared to 273 so close enough for your scuba tank calculation.