The waves in this woodblock print are sometimes referred to as tsunami (津波). In the caption they are called Okinami (沖波), big waves off the coast.
However, there are recent scientific publications that speculate that it was a tsunami wave after all.
The scene shows three oshiokuri-bune, fast barges that were used to transport live fish from the Izu and Bōsō peninsulas to markets in Edo Bay. The boats face south, probably towards Sagami Bay, to pick up a load of fish for sale in Edo.
The height of the wave can be estimated using the boats as a reference: the oshiokuri-bune were generally between 12 and 15 meters long. The wave is therefore between 10 and 12 meters high.
The whitecaps, as Hokusai draws them here, are considered an early example of the representation of self-similar structures, so-called fractals, for which Benoît Mandelbrot (1920 - 2010) developed the mathematical theory.