Good observation. Tidepools can be saltier than the ocean because of evaporation. We call it evaporation when water (or anything else) changes from a liquid to a gas. Things that are dissolved in water, such as salt, don't evaporate at the same temperature as water, so they are left behind. Now there's the same amount of salt, but less water to dissolve it in, so it's saltier.

Did you know that the Great Salt Lake in Utah is so salty that you float in it instead of sinking? It's so salty that the only animals that can live in it are brine shrimp (sea monkeys) and brine fly larvae. The amazing thing is that the Great Salt Lake is not the remains of an ancient ocean. It is fed by fresh water, which contains a tiny amount of salt. There's no outlet from the GSL; all the water that leaves evaporates, leaving the salt behind. In flood years, the salt content is about 5%. In drought years it's about 10%. Compare this to the 3% salinity of the ocean. How many times saltier is the Great Salt Lake?

Adding heat speeds up evaporation. Do you think that the saltiness of a tidepool will depend on the weather or the time between high tides? Do you think this is a problem for animals in tidepools?
Thanks for asking.