Yes The boy drowned in the river - this is ACTIVE - it was an accident or suicide - he probably fell in and drowned. The boy was drowned in the river - this is PASSIVE - somebody did it to him - he was probably pushed or held under the water by another person.

The boy downed in the river. He wanted to do this. The boy was downed in the river. Another one or another thing downed hin not himself.

These two are active and passive sentence. In the first one, the boy either drowned himself, so there's no one to blame for the accident. In the second sentence the boy was drowned by someone else.

The first sentence is simply about a boy who drowned in a river, with no futher details. The second implies that someone else drowned him in the river, a possible murderer. I'm not sure how to explain it, but in #1 the boy is an active subject, he's "in charge" of what happens, whereas in #2, the boy is a more passive subject that gets something done to him. If you extend the sentences, they could look like this; "The boy drowned in the river because he fell into the water." (boy is only active subject) "The boy was drowned in the river by his drunk step-father." (here, step-father is the cause)

Actually "was drowned" does make sense - it's either an older-fashioned way of saying the same as the first one, or in more modern English it implies that an outside party did it (the passive vs. the active) - i.e. "the boy was drowned in the river by the man". The first sentence implies he drowned by accident and the second could be either accidental or deliberate depending on the context, but it usually suggests that someone else did it.