Oh the machine gun toting robots and RC toys are in development. Amazingly, it's easier to fly an RC airplane, than it is to navigate ground based RC equipment over and around some obstacles.
That's all there is to it. Researchers are working on it though.
As far as water based RC weapons? I think there's just no call for it right now. We aren't fighting any navies, but there are ideas for using RC submersibles to disarm underwater mines. So, the tech is there, just no need to be using it.
There is a lag between command and actions. In the air, that lag is acceptable, and you have the advantage of being above the surface and easier to get a signal to the plane. On the ground, could be unacceptable. Speculating on sea, it may be tougher to get the signal to the "ship"
submerged RC stuff doesnt work. salt water stops high frequency radio signals needed to send data back and forth. the main issue is time over or at target area. flight can be extremely fuel efficient so time over target can be measured in days.
electric/solar power is ridiculously innefficient as is gas or other similar fuels powering ground vehicles. a surface support crew would have to be stationed fairly close.
ground bots are used at a unit level for recon but there is no way they can fill a role similar to a predator.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
An unmanned ground vehicle would lose the element of surprise, and render it useless. The predator is so sucessful because you can't see it coming, either by eye or radar; you can't fight what you can't see.
A RC tank would be just as susceptable to a RPG, mine or IED as a soft manned vehicle. We have had remote controlled underwater navy vessels for many years, they are called torpodes.
Full size ships would not make good unmanned vehicles, yet, because they are so dang complicated to run. While many systems on ships are completly computer controlled, it still takes a machinest mate to grease the shaft bearings, and a bosun to scrape the paint.
"It is better to be too skeptical then too credulous"