But before you buy something, if you want to find recordings of specific sounds, have a look at the various sample sharing sites. I've been able to find a lot of useful samples on them that are free to use in productions. E.g. here are some water sounds. Google will give you a lot of similar hits. http://soundbible.com/tags-water.html
Recording the sound of water is not as simple as one might think. I tried to capture some water falls and creeks in a mountain environment, where I expected good results due to the reflections of stones and rocks. The recorder was a Zoom H4N handheld recorder. Good device with good cardioid mics in XY configuration. But I just got something between white and pink noise, not a natural sound. It was unusable, the Logic Test Oscillator might have done the same ... I have not tried it again, but I can imagine that you need at least 2 omnidirectional microphones in A/B configuration or 2 cardioids in ORTF or something like that. And find the best positions of course. For best nature sounds, arrays of mics are used.
Unless you want to dive right now into the world of nature recording, you may follow bayswater's suggestion to look for samples first.
I have an iPad Pro with Garage Band so I could record audio to that, and use the transfer protocol between it and my new iMac (still to be purchased). But the in-built mic is probably not great. I will try it first and see what I can get anyway before spending any more dosh. Many thanks