No, you do not have anything to worry about here.
The leash loop insert is just like the footstrap inserts in the top of your
Unless you use too long a screw and somehow "punch out" the back side of the insert, the leash loop insert is completely watertight.
Be sure to use the screws that comer assembled in the nylon leash loop as they are the correct length for the insert in the bottom of your board. I believe they are shorter than regular footstrap screws, but I do not have my trailer on this trip so I cannot check that.
Hope this helps,