In the old days, it was a Princeton Tec bottom timer, depth gauge & tables.
These days, it depends where I am diving.
For shallow dives, not expected to exceed 30 feet with no diving planned for the next day, I just carry a depth gauge to confirm my depth.
For dives to less than 50 feet on air with no diving planned for the next day, I normally carry a depth gauge & watch then work off tables if need to do a second dive that day.
For dives to 100' on air or Nox with no deco planned, I carry a computer & a depth gauge/watch for a back up, so that at least I can time a safety stop at known depth if the computer fails.
For deeper dives & dives where I think that deco is a possibility, I bring the above rig plus a second computer. If the two computers start telling me significantly different things, I abort the dive. If they disagree by a minute or two, I go with the more conservative number.
...and a SPG with every rig
These days, it depends where I am diving.
For shallow dives, not expected to exceed 30 feet with no diving planned for the next day, I just carry a depth gauge to confirm my depth.
For dives to less than 50 feet on air with no diving planned for the next day, I normally carry a depth gauge & watch then work off tables if need to do a second dive that day.
For dives to 100' on air or Nox with no deco planned, I carry a computer & a depth gauge/watch for a back up, so that at least I can time a safety stop at known depth if the computer fails.
For deeper dives & dives where I think that deco is a possibility, I bring the above rig plus a second computer. If the two computers start telling me significantly different things, I abort the dive. If they disagree by a minute or two, I go with the more conservative number.
...and a SPG with every rig