Crossing into this country by sea is the path with most resistance. It is much easier to walk across a border than to paddle a boat (Not to mention your ability to hide is gone).
You are correct in that we cannot "seal" the border but, we could do much better than what is being done presently.
As of now, we have been 100% successful (nothing has happened)but, I can't attribute it to the great security on our borders.
Mexico is no friend to the US. Heck, their gov printed comic books telling its citizens how to illegally enter our country and get social services.
When the President says things like, "Mexico is our friend", I cringe... For obvious reasons.
Have you seen our coastline lately? There is nothing easier in the world than dropping a rubber dinghy off a freighter and motoring into shore.