In recent history, since WW2 really, military units move into a hostile country via the beach pretty often. So often that they created the Marines specifically to deploy in from outta the ocean damn quick, and having Marines gives a military a huge advantage.
The simplest, cheapest way to combat that kind of attack: mine the beaches. (Never mind that land mines aren't politically correct right now. You know they use them anyway.)
The new solution: robots. Robots who are marines! Have a swarm of robots land FIRST, coming from the ocean, and clear the beaches of mines. The robot shown above is "Ariel", a prototype that has successfully cleared mines in several demonstrations.
It operates differently for different situations. When if finds a mine, it can clamp on and wait for a signal to detonate, or it may prefer to try to pick up the mine and move it to a safer place. (The first choice is a suicide operation for the robot, of course...)
If it looks like a crab, that's because it is; it was completely modelled after a crab, so it could have the ability to walk over sand, rocks, etc. It understands that it has to change how it walks according to terrain, so a hole or a rock won't stall it (as we see in the first picture).
It starts underwater; it can withstand water pressure of up to 25 feet. It withstands the tides and lands on the beach. Or it might be deployed by a "lander" robot which carries a bunch of smaller robots to where they will be useful.
More info on this one is
here; if you go, there are several other awesomely cool military robot projects you can see, including a 3-foot-long mini-submarine that moves like a fish, and a robot medic that can inject immobile wounded soldiers with medication and apply bandages.
Did anyone see
Minority Report? Remember the spiders? I thought they were a little implausible, but this is exactly what the military is working on.