Funny though this is, it's all conditional on Luke implicitly believing Yoda when he says it ought to be possible.
Consider the analogous situation if he'd tried to lift the X-wing out by simply wading out there, getting his hands under one end, and heaving with his arm and back muscles. He might manage to get the air-filled cockpit end to lift a little bit, briefly, with its buoyancy to help him, but he'd pretty quickly be convinced that it wasn't feasible to get the whole ship all the way out of the water like that and furthermore that he risked putting his back out irreparably if he tried any harder. Even if the galaxy's best weightlifter and weightlifting coach was sitting beside him opining that it was a feasible lift, I think it would be entirely reasonable for Luke not to repose complete trust in that judgment, to think "that's very easy for you to say", and to still be unwilling to try it again, at least until he'd had a lot more coaching to build his muscle, train his technique and increase his confidence.
Yoda, of course, is more interested in training Luke to resolutely and competently oppose evil than to lift spaceships, since in most cases other than this one the latter can be done at least as conveniently with a crane; so rather than spend another few years of patient coaching on lifting heavy objects, he just lifts the thing out himself and gets back to more important matters.