The OTW system uses a centrifugal style pump with a slinger ring. According to the manufacturer, they can pump upto about one atmosphere, or about 15 psi.
That's enough to pump against a depth of over 30 feet, so that's enough of a safety margin in my book!
The OTW system displaces air into the dry areas as the tank fills with water. This pressurizes the cylinder slightly to a few psi (will vary but probably around 4-6psi), so if the tank is filling okay, that suggests the pump is working well enough. You say the WTC is fully watertight, but is the tank air tight? If air is escaping as you pressurize, then when it comes time to empty, you will be pulling a vacuum, and I'm not sure how well the pump works against a vacuum.