Don't rely on a base tan to prevent burning. It's equivalent to a max of SPF-3.
For the curious:
The sun protection factor of a sunscreen is a laboratory measure of the effectiveness of sunscreen the higher the SPF, the more protection a sunscreen offers against UV-B (the ultraviolet radiation that causes sunburn).Sunscreen - Sun protection factor (SPF)
The SPF is the amount of UV radiation required to cause sunburn on skin with the sunscreen on, as a multiple of the amount required without the sunscreen. There is a popular oversimplification of how SPF determines how long one can stay in the sun. For example, many users believe that, if they normally get sunburn in one hour, then an SPF 15 sunscreen allows them to stay in the sun fifteen hours (i.e. fifteen times longer) without getting sunburn. This would be true if the intensity of UV radiation were the same for the whole fifteen hours as in the one hour, but this is not normally the case. Intensity of solar radiation varies considerably with time of day. During early morning and late afternoon, the sun's radiation intensity is diminished since it must pass through more of the Earth's atmosphere while it is near the horizon.
In essence, SPF determines the multiplication factor based on a specific level of UV radiation.
One factor that is of vital importance, from the same page...
The SPF is an imperfect measure of skin damage because invisible damage and skin aging are also caused by ultraviolet type A (UVA, wavelength 320 to 400 nm), which does not cause reddening or pain. Conventional sunscreen blocks very little UVA radiation relative to the nominal SPF; broad-spectrum sunscreens are designed to protect against both UVB and UVA. According to a 2004 study, UVA also causes DNA damage to cells deep within the skin, increasing the risk of malignant melanomas. Even some products labeled "broad-spectrum UVA/UVB protection" do not provide good protection against UVA rays. The best UVA protection is provided by products that contain zinc oxide, avobenzone, and ecamsule. Titanium dioxide probably gives good protection, but does not completely cover the entire UV-A spectrum, as recent research suggests that zinc oxide is superior to titanium dioxide at wavelengths between 340 and 380 nm.
Best bet: avoid prolonged exposure, if possible. It is also important to be aware that on a beach or in a pool, not only is one exposed to direct sunlight, but also to reflected light from the water. This can substantially increase the UV intensity.
Be careful, be safe, and have a great time!