Rain catchment is a good way to top-up your tanks. Some sailors set up tarps to catch and lead the water into Jerrycans, others have the water flow directly into their main tank — I don't recommend doing that unless you've got a good filtration system. When it rained, we'd collect water trickling down from our aft solar panels into a 20 liter bucket. We collected this extra water for showers, laundry and washing dishes.
While the rain itself might be safe to drink, it carries dirt or whatever may be on your tarp, or deck. And depending on where you are, the rain may have environmental pollutants mixed in. For these reasons, I recommend using rainwater for purposes other than drinking and cooking. If you are to drink it, it's important to boil it first for at least 1 minute, or to run it through a good filter. See water filtration.
Even if you don't plan on drinking it, it's a good idea to treat the water you catch. The easiest way to do this is to add chlorine bleach to your supply. It is the most reliable treatment method. This is something we've done, and continue to do. The suggested ratio of chlorine to water is 2 drops bleach for 1.15 liters (1 quart). The above ratios are for bleach containing 5.25% Sodium Hypochlorite. This amount depends on the concentration, the ratios will be different for 5.25% than for 8% chlorine bleach (1 drop: 1.15 l). Only use regular, unscented chlorine bleach products that are suitable for disinfection and sanitization, as indicated on the label. A good way to remember these ratios, is this saying:
“You must be 21 to drink”.
2 drops of bleach per 1 liter (quart) water, easy to remember. Note that 1 quart is slightly less than 1 liter.