The rooms at Casa Rosario are dark, dank, and not the cleanest by developed world standards. The "solar water heater" for the shower (this is what Guatamaltecos call it when the water tank is on the roof) left a little something to be desired, but at approximately US$ 2.50 night (at the exchange rate in Spring 2000), that seemed ot be the going rate at every posada in town that catered to the budget traveller. If you want a freshly painted white room with a view of the lake, you naturally have to pay more.
There is a clean kitchen in Casa Rosario where you can prepare your own food purchased at the mercado or one of the many family-run tiendas in the neighborhood. I forewent the trickling water from the "solar heated" shower, and opted instead to go down to the lake every morning with my shampoo. Here I was politely ignored by the Mayan women who were there with the same idea. (Yes, they bathe with their clothes on.) When you leave Casa Rosario, make a left out the door and then a right at the bottom of the road away from the Police Station. Continue past the school and look for a footpath on the left. This will take you to the cleanest stretch of black sand beach I found on the lake.
Because I was a budget traveler, I was quite content with my accommodation there: My fellow boarders were all as friendly and as mellow as could be, and I could theoretically roll out of bed for my Spanish lesson. (This was never the case - every morning at 6:30, the farmer across the street would begin work by revving up his rickety, gas-powered molino to start grinding coffee.)
If the rooms are not up to your personal standards, I still strongly suggest checking out the Spanish lessons here (see my free form entry below).