Don’t be a lifeguard if you’d have trouble with a 550yd swim! If you’re determined the best practise is to just get in a pool and do it. That will not only develop specific fitness but also give you what swimmers call ‘feel for the water’.
Also if you know any good swimmers then get them to watch you for a couple of lengths and critic your stroke. If you’d have trouble with this distance its probably mainly a technique issue which can be solved easily. You’ll find that improving your stroke even slightly will have huge impact on your swimming abilities. Far bigger than any fitness gains you might make.