Located about 1km from Cascais, it's an easy walk (there's a flat path that hugs the coastline all the way from the marina). Leading up to the Boca do Inferno, there are a series of cliff faces and rock formations being battered by waves, which give you a taste of what's to come.
The Boca do Inferno itself is one of those curious naturally formed rock formations that couldn't have been hand-sculpted any better. It's effectively a lagoon with almost vertical rock faces surrounding it, save for an archway (the boca/mouth) at one end. This acts as a funnel for huge waves to come crashing into the lagoon, which creates a swirl of white water. We were mesmerised for 15 minutes, waiting for the next big wave to come crashing in, while marvelling at the birds that would swoop through the archway and perch just out of reach from the incoming waves.
If you wanted to sum up the power of the ocean in one place, the Boca is a pretty good candidate.