As everyone says, clarity of the water makes a big difference, and was a big variable.

In general it was hard to spot a submerged boat, but under good conditions you might see the boat when it was just under water (ie: persicope depth). Under ideal conditions a bit ddeper. 60 ft is a bit below persicope depth for a VIIC, so this would be near the outer region, but not the absolute limit, of when a boat might be spotted, though under most conditions 60 ft would be too deep to be seen.

It was relatively rare for an aircraft to spot a fully submerged U-boat (no persicope or shnorkel showing).

In the Pacific, where the water could be clearer and many sub actions were in shallow waters, US boats were often spotted, and bombed, by aircraft while submerged. Of course, this many times was due to the periscope wake being seen, calling attention to the faint, shadowy outline of the submerged boat. But there are a number of cases where US subs were spotted and bombed (or depth charged) from the air as deep as 120ft. It certainly was unnerving to be making a quiet, submerged approach on a ship with nary an escort in the area and suddenly, WHAM!, a depth charge goes off nearby, dropped from an airplane no one knew was around.....