is there any generalizing word to call lakes, rivers, ponds etc?

Even just “the water” will work. Like the first picture “the couple are walking towards the water” .

The generalized term is "body of water". This would also apply to oceans, seas, etc.

bodies of water

There is a term "waterways", though I believe that covers rivers and canals, but not ponds/lakes/sea. For something like the 1st picture I'd use "the couple is walking by (or on the shore of) a large body of water".

Body of water! However, I'd guess that that's a lake, since it seems too big to be a pond (can't see the other side), and there doesn't seem to be sand at the edge like an ocean beach

You could use body of water, but it would be somewhat strange to say “the couple holding hands walked towards the body of water”. For the assignment I would just pick something and use it: “the couple holding hands walked towards the lake. The wind caused the lake to ripple. The couple saw hundreds of ducks on the lake.” Etc

The general term is body of water. The fact that we lack a single word for this is telling. we almost always use the more specific word.

I would use the word “shore.” It is a generic term for the edge of a body of water, and a more appropriate term for your assignment, in my opinion. 

"Bodies of water"

Body of water

It's called a *hydronym.* It's a type of toponym that designates a proper name of a body of water.

"bodies of water" "waterways", although that typically applies to navigable waters.

"Body of water" is the term you're looking for.

You could just refer to "blue features". Blue feature being old military slang for any body of water as they are all shown thus on maps...

I would say "bodies of water"

Bodies of water

"body of water "

If you also had swamps, marshes, fens and bogs they would collectively be called wetlands.

If the context is clear, people often just say “the water” rather than “body of water.” Let’s go for a walk down by the water’s edge. I love being near the water. The water always makes me feel calm and relaxed. Can we go boating on the water?

A Waterway would work, but it's a bit archaic. I would say "sea" in this regard as you can't clearly see the other side of the body of water, and I tend to find Occam's Razor works best in this regard (Occam's razor says that the clearest answer is generally the correct one).

we have the word "shore." the land along the edge of a sea, lake, or other large body of water.

Lakes, rivers, and ponds are generally “freshwater”, as opposed to oceans (“salt water”).

You could just say "they're walking near a body of water." A body of water can be anything from ponds to oceans!

'waterbody', if you're looking for a single word.