Even as brooks and streams across the drought-stricken Midwest slow to a trickle, water investors are swimming in profits.

More than four years have elapsed since I highlighted the deep-value, long-term investment opportunity in shares of Aqua America (NYSE: WTR), and the company's release of second-quarter earnings this week provides a convenient chance to dive in and test the waters of the water utilities once more. Before we do, have a look at the following chart, which tracks the performance of major water utilities (from the time the above-linked article appeared to the present) against the S&P 500 (INDEX: ^GSPC) and the PowerShares Water Resources Portfolio (NYSE: PHO) exchange-traded fund.

WTR Chart

WTR data by YCharts

As you can see, the water utilities have certainly been the slickest way to gain investment exposure to water over recent years, and I expect that trend to continue as the top names in the space sustain the same winning strategy of "growth through acquisitions, investment in infrastructure, and the recovery of cost increases and investments through rate increases" that I highlighted back in 2008. Aqua America shares have gained 34% over the period, but total gains soar beyond 50% once dividend payments are factored in. That's hardly the sort of underwhelming trajectory one might expect from a defensive dividend play of the sort! Larger rival American Water Works (NYSE: AWK) has fared better still, enjoying a remarkable 77% nominal gain! The water ETF, meanwhile, with only 25% allocation to the utilities, remains underwater and gasping for breath.

During the second quarter of 2012, Aqua America grew income from continuing operations by 16.5% over the prior-year period. Earnings per share climbed 11% to $0.30, corresponding neatly to an 11% increase in revenues to reach $198.2 million. Those robust results were driven both by newly acquired assets and by continued success in recovering the costs of needed infrastructure investments through approved rate increases and surcharges. Broadly speaking, ongoing efforts to rebuild our antiquated water infrastructure form a piece of the great infrastructure imperative that steelmaker Nucor (NYSE: NUE) discussed in its latest earnings report.

To help ensure that investments like Aqua America's 2012 capital expenditures budget of $300 million continue to drive shareholder gains, Aqua America has swapped assets with rival American Water Works and others in order to focus its growth agenda within "states that demonstrate a positive regulatory environment."

Parting thoughts for thirsty investors
Although I believe the top names among U.S. water utilities will continue to deliver low-risk, long-term gains, I personally would not be looking to build a full position all at once now that these stocks have surged dramatically in the past couple of months. As I do for most of my stock purchases, I would look to build a position gradually in hopes that better entry points might present themselves. Second, while Aqua America earned my top nod in the space back in 2008, that discussion pre-dated the April 2008 IPO of American Water Works. My Foolish colleague Sean Williams has made some compelling arguments in favor of American Water Works as the prime water utility, and I encourage Fools to dunk their heads into both quality operators before taking a plunge into one or the other. And finally, if you're drawn to water by a thirst for strategic exposure to commodities in general, then please bookmark my article list or follow me on Twitter to keep informed with my ongoing coverage of the commodity complex.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.