We now know that the SPF in a Sunscreen stands for 'Sun Protection Factor'. A product thats provides 30 SPF means that you can stay outdoors 30 times longer than you could with no sun protection. Keep in mind however, everyone burns at a different rate.
The buzz word in the sun protection arena is UPF, 'ultraviolet Protection Factor' rating in fabric developed by the Australian Radiation Labortory.
eg. Lycra which has an elastic thread enables the fabric to stretch and bounce back (memory). It is woven with nylon yarn, then dyed and softened. The Australian UPF rating goes up to 50+ protection.
Oiwi Waterwear has a light weight 4oz lycra knit that is 100% UV protection, fast drying, breathable and super soft.
The protection may be lessened by the fabric being to close to the skin, if the fabric is stretched, is wet, or worn out.
Rash guards are necessary due to skin abraisions caused by chaffing, but keep in mind, a white cotton t-shirt provides only 5% UPF protection (less when wet)
Remember a garmet only protects the skin it covers. Protect other areas by wearing a cap with a four-inch bill, UV protective sunglasses and sunblock.
With all those areas covered, you litterally will save the skin you're in.