Just about any decent surf site has one.
On the great Magic Seaweed
or non surf site
in fact there are quite a few.
However, they are little guide to temperature over the beaches, which are so dependent on whether the wind's onshore (warm) or offshore cold, as well as whether the sun's out. I've recorded 20C off the Battery Rocks, Pz, one day during a sunny afternoon with an onshore breeze piling in the warm surface water, near 15C the next in a moderate NW wind with upwelling.
This is a good site for SST inshore, even these can show some quite large day to day variations.
using Penzance as an example
Also, there a lots of buoys out there of course.
A very wet & windy Penzance - perhaps some thunder at last?