Boats in/at/on sea

"on the sea" implies on the surface of the sea/ocean, so ships cruise on the sea.

"in the sea" implies being "in" the water, so fish swim in the sea.

You can treat "at sea" as an adverb, much like "at home" or "abroad". Here are the verbs we usually use with "at sea":
-be at sea
-lost at sea
-buried at sea
-die at sea
-dumped at sea
-missing* at sea
-

Ok here is a real life example:

1."Can ships track other ships [at/in/on the sea] using a radar?"

2."Can ships track other ships [at/in/on the ocean] using a radar?"

I think most people wouldn't bother saying where the ships are:

"Can ships track other ships using a radar?"

Where else are the ships going to be? Most listeners/readers would know from the context where the ships are. If you really needed to specify where the ships are, you could use "at sea":

"Can ships track other ships at sea using a radar?"

If you u

MustAsk1."Can ships track other ships [at/in/on the sea] using a radar?"2."Can ships track other ships [at/in/on the ocean] using a radar?"

Where did you find them ? This is far from being a good a live example. A native speaker will never speak in such pattern.
Ships can see other ships (in the same water) on radar. Tracking - infers following with a pu

grammarfreakThis is far from being a good a live example. A native speaker will never speak in such pattern.

Not many native speakers will produce sentences like those

grammarfreak Tracking - infers following with a purpose.

Do you mean 'implies'?