Even when I use waterproof sunscreen, I still reapply sunscreen when washing my hands. If you read the back of most waterproof sunscreen, it instructs you to reapply after getting wet or toweling...so I try to keep lotion with SPF next to the sink to remind me.
I've read that skin sags due to loss of collagen. Collagen degradation occurs naturally as we age as well as from the sun. So you are correct in thinking sunscreen can help prevent sagging skin. In addition to prevention, you can use products to promote collagen synthesis. Vitamin C (L-ascorbic acid) is a skin care ingredient with much research to support its collagen promotion abilities. An excellent Vitamin C products is Skinceuticals C E Ferulic, but it cost over $100 dollars
If you don't want or can't spend that amount, you can try to make a product similar to it for less than $20.