Getting a 2500k over a 3570k (if prices are similar) makes no sense if you are water cooling. With water cooling, you are already handling the higher heat issue of the i5 3570k, thus making it ideal to get a 3570k as it is already ~10% faster than a 2500k. Plus, there is not a 1 for 1 relationship with overclocking. For example, a 10% overclock does not necessarily yield an exact 10% increase in performance. So, you will have to overclock that 2500k between 10% - 15% to get comparable performance to the 3570k at stock speeds. I have a 3570k and on the stock cooler can run it oc'd to 4 ghz and never hit above 72 Celsius on my hottest core and am usually in the 60s while gaming.
In other words, the heat will not be a major factor for either chip if you are water cooling, so why waste some of the performance potential by getting a 2500k that will right out of the gate need a 10% to 15% oc just to match the 3570k, especially when the 3570k can oc up to ~5 ghz with proper cooling.
Also, don't overlook getting a good motherboard that is friendly to overclocking. An ASUS Z-77 V Pro or above should get the job done without breaking the bank. For example, you can pick up a 3570k for about $189 + tax at MicroCenter if you live near one. They also give $50 off for a motherboard if bought in combo.