I myself will change a light bulb, but beyond that I call an electrician on the theory that they're cheaper than hospitals or funerals.
You always want to avoid extension cords or any temporary makeshifts when using electricity outdoors. You may find, of course, that your waterfall or decorative light or filter (I use none of these, but issue this warning for those who do) comes with a little short cord meant to plug in beside the pool. It is against the law, however, to have an electrical box closer to a lily pool than 10 feet in Washington, so the problem arises how you get the four-foot cord to reach the box 10 feet distant.
The usual solution is an extension cord. You might think that since one great purpose of the law is to discourage extension cords, you should be able to install the electrical outlet three feet from the pool so your cord would fit nicely, but this is not how the law works.
The danger of water around electricity is so great that the law insists on 10 feet of distance between the two. It makes no great sense for gardeners with lily pools, faced with an extension cord no matter how many electricians they hire, but you have to remember the question of swimming pools. The aim of the law is to prevent makeshift cords for radios, say, that careless people would almost certainly drop in the pool sooner or later.
Hence the law requires an electrical outlet no more than 15 feet from the pool but no closer than 10. The theory evidently is that if you have an outlet 10 feet away, you'll plug your radio into it rather than hook it to some feeble extension cord, and this 10-foot distance will prevent your dragging your radio close enough to the pool to drop it in.
Fish pools are treated as if they were swimming pools. Otherwise, I guess, people would say their swimming pools were fish pools and thus dodge the law.
The point is, get your electrician out to advise you before you start building a pump house or fiddling with waterfall motors. You will save much spinning of wheels if you work out the electrical problems first, and decide on the location of the motor afterwards.
As in much else in gardening, it pays to know what is involved before you start. If the pool is some distance from the house you will run into the easily solved (but vital) problem of how to get the power to it. Sometimes the cable can be run overhead, through trees or along fences. It seems to me obviously safer to have the cable buried, so dogs and squirrels won't gnaw it or tree branches fall on it. If it is underground, the law requires it to be 18 inches beneath, so don't count on doing it with your trowel or just laying it on the ground beneath shrubs.
Naturally if you have the 18-inch trench dug (having checked with the electrician first, to make sure you aren't running the trench in the wrong place as far as he is concerned) you will save the quite considerable sum you will otherwise be charged for the labor of digging.
In electricity as in everything else, nothing is as simple as the gardener thinks it will be. Running a line from the house sounds easy enough, but you will probably find a cable has to be run along the basement ceiling and two masonry walls may have to be drilled to carry the line outdoors to the trench. Before the line gets to the trench it may have to be enclosed for a distance within a pipe. You may have to build something 10 feet from the pool to fix the outdoor box to. Little things of this sort will run the bill up nicely, but not nearly as much as if you barge ahead and then discover, when the electrician comes, that you have things in the wrong place and have to start over.
I think waterfalls are dumb, and so are fountains, but since many people insist on them, I am trying to save you time and money by urging careful planning before the actual work begins. As the bill mounts, you will want to keep in mind that the very best electrician is still ever so great a bargain compared to funeral costs and all that inconvenience to everybody.