From my limited experince with landing craft, there may be problems with steerageway, & drift off course, from the boyancy/high center. Also suspected is making way against the winds & currents of the open sea will raise fuel consumption over that of the a river or cannal passage.

Finally in the case of Sea lion I am wondering if the Germans really had enough seamen experinced both in handling boats on open waters and in handling the barges. From the several descriptions of their 'exercise' I've read one would guess not.