I'm a pathologist, a medical doctor who probably uses a microscope the most out of any physician and although I can see some potential upsides - even I would put this somewhere near the bottom of the list of things to keep on hand in a realistic prep setting. Preparing, staining and interpreting a slide are not trivial skills for a novice in a resource-limited environment.

This sounds like a simple, straightforward problem that you could solve with a microscope, right? I have an infected wound, and I want to know what's growing in it. Just do a wound swab and make a gram stain to see if you're dealing with gram negative or gram positive bacteria, to help guide antibiotic choice (especially if you've got a very limited supply). Or, as you mentioned in the original post - I'd like to see if my water is contaminated, so I'll check it with a Gram stain. But even performing a Gram stain properly in a resource limited environment is a challenge.

And let's hand-wave that fact, and assume you've made a proper Gram stain that isn't contaminated, over or under stained - seeing bacteria requires a well-calibrated microscope. Do you know how to set one up? You can barely see bacteria well at 40x and, as a novice you'd ideally like to see them under 50x or 100x - both of which require oil immersion lenses, unless you've got a specialized and even more expensive "high dry" objective. Do you know how to care for and maintain a microscope with an immersion oil lens? They tend to get gummed up quickly when novices use them. And careful, if you break a lens, you're not easily going to find a replacement.

The reagents required to make what most would consider an extremely basic/simple Gram stain are not trivial to find, and certainly not trivial to replenish in a resource-limited environment. If you run out of, let's say, crystal violet stain, or you knock it over while preparing a slide - you're going to be up a creek. Other stains and preparations would require similarly difficult to stock/replenish reagents. An acid-fast (mycobacterial) stain, wright-giemsa stain or a roughshod hematoxylin/eosin stain would be beyond the scope (ha, no pun intended) of basically any resource-limited environment for a novice or untrained person to prepare.

And let's assume you've somehow mastered the staining technique - are you experienced at identifying what you see? A common phrase in pathology is - "the eye can't see what the mind doesn't know". You could be staring at an obvious deadly pathogen, but if you don't know what it looks like - you're still going to be navigating blind. Atlases can help, but they still only give you a handful of examples. Could you tell me what an obvious Corynebacterium colony would look like under the scope, and which antibiotic you'd choose? What's a more common bacteria that could look similar and trick you under the microscope? What are some key features to distinguish it from similar looking gram positive rods? Do you know how to tell if you improperly stained the slide, so a gram negative bacteria erroneously appears like a gram positive? What about the reverse, how to tell if you improperly prepared the slide, so that a gram positive bacteria looks like a gram negative one? Are you confident your staining technique ensured that you didn't contaminate your slide with external sources of bacteria? Is that Staphylococcus from your hands, or is it growing inside your water containers?

I don't say any of this to discourage you from getting interested in this stuff, and learning it now. It doesn't take much to get comfortable identifying gram negative and positive bacteria, and if you are going to be considering keeping a microscope and reagents on hand -- if you learn how to use it now, you'd be ready to use it later. But I would strongly advise to spend your time and money elsewhere if you're not going to use it properly. Scopes and reagents are expensive, and you could put that money to good use for other preps!

I do have all the necessary components on hand to make and utilize a gram stain, but, that's more a byproduct of my profession and training than it is a deliberate prep! I hope never to have to rely on this aspect of my training in my day-to-day life.