Re: What is an undertow and a riptide? Are there any other ocean hazards?

Riptides and undertows are related. Breaking waves approaching the beach carry water toward the beach. The water can't just pile up there: it has to escape back out to sea somehow. If there's a place along the beach where the waves aren't as strong, the piled-up water near the shore escapes through that weak spot, flowing back out to sea. This is a rip tide. If there is no spot with weaker surf, the piled-up water flows down and under the waves and back out to sea, forming an undertow.

I've never experienced an undertow which was strong enough to actually suck someone under water: most deaths attributed to "undertow" happen when people playing in the area where the waves run up onto the beach get their feet knocked out from under them when the water flows back down to the sea. They get dragged a short distance into the breakers, and aren't strong enough or knowledgeable enough to get back to shore.

To escape a riptide, swim alongshore to the left or right, to escape the seaward-flowing jet of water. Riptides can flow much faster than you can swim: if you try to swim directly back to shore against the current, you'll soon become tired and risk drowning. I've been caught in riptides several times while surfing: in fact, surfers often use them as "express lanes" to get back into deep water after catching a wave. But don't do this unless you're a strong swimmer, have a surfboard for flotation, and know exactly how far out the riptide will take you.

I can't find any information which suggests that whirlpools like the Maelstrom of Norse legend and the Charybdis of Greek legend actually occur in sizes large enough to sink ships. I have good reason to doubt that they exist: in order to get a compact, very strong spinning mass of fluid, like a bathtub whirlpool, a tornado, or a hurricane, there must usually be some form of suction to concentrate the "vorticity" (spin) of the fluid at a single spot. For example, in your bathtub, as the water flows toward the drain it carries any spin you've caused by sloshing in the bath toward the drain. The vorticity in the whole tub gets concentrated at the drain spot. A hurricane behaves the same way. I don't see any mechanism to concentrate spin this way in the ocean. You might check out this website, which talks about some norwegian scientists' search for the Maelstrom. Their computer model generates strong tides, but only weak eddies (spinning motions) at the legendary location of the Maelstrom. There's also a recent Mad Scientists' Network answer which talks about whirlpools.

Aside from storms, probably the biggest sea disaster is the tidal wave, or "tsunami". These giant waves, often 50 feet or higher, are generally caused by earthquakes or underwater landslides. They can travel across entire ocean basins and completely destroy coastal cities. In some places in Hawaii, where I grew up, you can still see signs of the tsunami of 1960.