Paul, Simon,
pumping some air cushion in can help a bit, but only when the boat lies quietly on even keel. But when the boat is heeled, the air blows out through a bailer before all the water is pressed out. We experimented with air years ago, but found it was not worth the extra complexity of fitting and connectiong a pump for it, and there are further serious complexities from pumping pressure into the tank, at some point you may easily cause a crack and leak where tank and hull are bonded and sealed together. If that happens you'll regret it.
Not recommended.