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From this question we know the barge can be held at a set position with an accuracy of one meter or so. So the barge doesn't really move in the X and Y direction. This takes care of yaw as well. But that leaves 3 degrees of freedom: moving in the Z direction (up/down), pitch and roll.
None of these can be easily compensated for with the current barge design.

Does the stage have features that would help compensate for this motion? As the deck can't be counted on to be horizontal, you run the risk of having one leg touch down first. For the Jason-3 mission, the landing attempt failed because the lockout on one of the legs failed: would this lockout be applied before landing, or after?
Doing it before landing gives the opportunity to use the hydraulics to adjust the angle of the legs, at the cost of making the hydraulics more complicated.
Compensating in the Z direction would require a landing altimeter on the stage to get accurate height readings just before landing (GPS being less accurate in the Z direction).

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The barge is fairly long (300 ft?) and is held pointed into the waves. As a consequence for it to pitch significantly, it needs a wave longer than itself. It is rare to have waves that large in conditions that are safe enough to land.

Thus it mostly rides the crests, and apparently stays fairly stable.

The legs have some bounce in them, as has been seen in the two landings, and can accommodate the slightly moving deck.

In the Thaicomm-8 mission we saw the landing come in at something of an angle, and over extend one of the legs. Elon tweeted that there is an aluminum honeycomb crush core, used as a contingency to handle excessive movement. It also means it does not recover back, as the list of the landed stage showed as it came into port.

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    $\begingroup$ The first stage is also very bottom heavy so it tends to stand upright more easily. $\endgroup$ – Sarah Bailey May 12 '16 at 20:43
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    $\begingroup$ As @SarahBailey said, it has a low center of gravity, which should probably be called a center of mass when talking about spacecraft (as opposed to other vehichles humans use) where a spaceship "often" operates in realms where gravity is not as much of a concern as it is near the Earth. $\endgroup$ – FKEinternet Jul 14 '17 at 23:06

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