The MOD for 100% O2 is 20 feet. If you're going by the 1.4 PpO2 limit. If you go by 1.6... At the site in question, it's pretty much impossible to get below that. And keep in mind that the MOD is for a sustained (but not precisely defined) time deeper than that depth. It's not like you're going to instantly die if you do wiggle down into a depression and hit 21' for a couple minutes. You would have to reach 21.5 feet before instant death occurs. And of course, if you get as low as 22 feet, your entire family, including pets, will undergo spontaneous combustion.
The point is that with a high octane breathing gas, the OP will still be off gassing on a 20 foot dive.