Fair skin tone must choose higher SPF in order to get longer protection (the amount of protection you can). Be aware that the numbers behind SPF explain when you need to re-apply SPF.  

Stay on the recommended use of sunblock to avoid damage the next day. The skin will begin to burn if it has been exposed in the sun for 10 minutes that has been proven by science. 10 minutes become the benchmark numbers on the SPF. For example you have a product SPF 20 x 10 min = 200 minutes of protection time. The higher the SPF the longer it protects your skin

Mixing different SPF products adds protection time from sunscreen. Suppose you are using SPF 20 moisturizer, SPF 40, and SPF 30 Powder, add it to SPF 90 x 10 = 900 minutes. This understanding is false, SPF does not work in that way. The protection will be based on highest SPF product which you have applied

UVA rays penetrate deep into the sea. It is recommended to use Sunscreen Water Resistant for those who like to swim and walk to the beach. There is waterproof sunscreen in the market but it is not waterproof for 24 hours. The usual water resistance can hold up to 40 minutes under water and sun. Longest waterproof time is about 80 minutes. Be sure to re-apply when you are in the pool / beach