Most stand-alone UV units are supplied with fittings to mount to a vertical or horizontal surface so that you can install them in the most convenient location externally. In the photo, it's the long white unit.

But if you don't have external pipework, there are units that provide the option of being submerged in the pond if required.

Some units are designed to fit on top of the black-box type of filter used to keep garden ponds clear. Others are designed to deal with the larger gallonage and higher stocking levels of enthusiasts' Koi ponds.

In addition to price, look at specific features, recommendations, claims for longevity and ease of changing lamps, and ask fellow water gardeners for their experiences, especially in maintenance.