Water is basically reverse slime: as you match dots that are touching the water, you’re cutting away the background to reveal more water. Usually the goal is to clear every piece of background in addition to something else — ice, anchors, beetles, whatever.
The very first thing you need to do is see where the water originates on the level and figure out how quickly you can open up that water source. Determine if you’re going to need to use water to hold back slime, and then see if you need to handle anchors. Prioritize everything in your head and then have at it. Water’s not quite as difficult as slime, but again, if you need to restart because you know there’s no way to clear the level, there’s no shame in that.
If this guide helped you out, please consider buying one of my stories from Amazon. Here’s one: Amid the Steep Sky’s Commotion, as a reprint in Beyond Science Fiction. This one’s a steampunk murder mystery with a love story b-plot.