The amount of splash a swimmer makes when kicking does not indicate how hard they are kicking, nor does it indicate how strong their kick is. The splash in a swimmers kick is caused by the feet coming out of the water and then entering again, especially in freestyle.
Catching too much air can be seen as wasted effort and can also lead to a swimmer fatiguing very quickly. What we strive for in swimming is a solid and sustained kick without too much wasted effort in the splash. Also a strong kick will help the underwater work that is needed for good starts and turns.
We have begun to introduce our swimmers to vertical kicking drills, as shown in the video below. These drills are best performed for about 10 seconds with 10 seconds rest with 3-5 repetitions in the first instance. The heart rate will rise very quickly and the legs will fatigue rapidly, as water is being moved in both directions of the kick creating resistance.
I recommend introducing swimmers to this drill with fins in the first instance before progressing to vertical kick without fins once enough strength has been established to perform the vertical kick properly without the head sinking below the surface of the water.
Have fun, train safe and train smart!