In all the discussion about ETFs that track energy, resources and related fare, the ETF Professor started to wonder about water ETFs and if they offer any opportunites for investors.

Water is kind of stodgy investment in the vain of industrials or utilities, but it is clear that there are water shortages all over the world and many emerging markets lack adequate access to fresh drinking water.

So from a long-term perspective, water ETFs may be worth a look. The PowerShares Water Resources (NYSE: PHO) is the most liquid (no pun intended) name in the group of four water ETFs.

That said, PHO is deeply correlated to the PowerShares Global Water ETF (NYSE: PIO), which offers a bit more of an international bias. The correlation between PHO and PIO rests at a startling 96% so there is really no point in owning both ETFs.

The Claymore S&P Global Water ETF (NYSE: CGW) and the First Trust ISE Water ETF (NYSE: FIW) offer similar exposure to PHO and PIO, but are thinly traded.

So the Professor says go with PHO, but only as an investment not as a trade and only allocate a small percentage of your portfolio to it.

An arguement against PHO, or any water ETF, is that an industrial ETF may offer better returns as water is essentially a niche play on the industrial sector.