There are many levels of SPF out there and I would say anything lower than SPF 30 is not worth it at all. Think of how our hair provides essentially SPF 15 so you need 30 and above for everything.
We are all guilty of not doing this at some point. So ideally we should be reapplying our sunscreen every 2 hours and more frequently if we are in the pool or in an environment where the sun is very strong.
I know not all mineral SPFs are good. Some can be super dry and sticky, but overall better choice and reef friendly. Unlike chemical SPFs no need to wait 15-20 min to apply before sun exposure.
Sun protection doesn't just stop at sunscreen. Clothing and accessories can also be an SPF. You should have sunglasses and a hat with you on these hot summer days.
We forget to protect our lips from the sun as there aren't many lip balms out there with an SPF. I feel like every lip balm or chapstick should just come with an SPF so you never even have to think about it.