A couple of thoughts...
100% O2 at 50 or 60 feet will not kill you immediately. The deeper you go, the higher the partial pressure, the faster you will tox. In addition other factors come into play such as temp, activity level, CO2 levels, etc.
So a short exposure at 30 feet may not kill you right away but your risks increase the deeper you go and the elss support you have.
It was not uncommon in the past for commercial divers to be brought up from their 40 ft stop and then be blown back down to 40' in a recompression chamber while breathing 100% o2 through a demand mask. The thing is air breaks were used to prevent ox tox and in the event symptoms of an incipient ox tox were felt (tingling lips, mouth, visual distortion, etc) the diver would remove the O2 mask and breath air from the chamber. In the event he convulsed with no warning symptoms, the hand held mask would fall off anyway and the PPO2 would again immediately drop.
In the water it is a different story as the reg gets spit out and you drown when you convulse. Some in water recompression protocols call for O2 at a 30' stop, but as such they require a bosun's chair to assist the diver in maintaining depth, a full face mask and a monitoring diver in attendance to deal with any ox tox issues that may arise.
Deep air record dives by definition involve short exposures at extreme PPo2's and the US Navy has even developed a chart of acceptable (and more or less safe) exposore times at high PPO2's, but they are very short and in general get much shorter as the PPO2 increases.
So it is not so much that a PPO2 of 1.6 is safe while 1.7 will kill you but rather that anything over 1.6 is not considered prudent due to the shorter exposure times allowed and the greater varaibility found in individual diver suceptibility at those higher PPO2 levels.
As an aside I read a wonderful account recently by a diver who participated as a virtual lab rat in some rebreather experiements in England during WWII - when they had no real idea of oxygen toxicity and how it worked. Generally they found that it varied but in a specific case, when placed in a pressure tank under water divers toxed after about 10 minutes at 50 feet on 100% O2. Scary stuff. Scarier still when you consider they kept doing it until they figured out what the problem was.