I think of a surge as a larger undercurrent that flows beneath the waves. You'd have waves in regular frequency and amplitude, then a larger "wave" which lifts all the rest of the waves upward, which I consider a "surge". Kind of a large wave that carries all the waves. I think some consider it to be any kind of water movement that behaves like a wave.
Surge is a back & forth motion of the water of a few meters or so. It's usually caused by wave action and occurs near shore. It can be fairly violent and the biggest problem is the risk of bumping into things. Some divers also get seasick underwater from the motion. You manage surge mainly by leaving yourself more distance from obstacles or delicate reefs. If the situation allows you can move away from shore and surge will be reduced.
Surge is moving back and forth due to wave action above.
Surge, usually in the shallows and caused by wave motion overhead especially when the sea bed is funneling it. Its an up/down main motion.
don Francisco and perdidochas are close in their description of surge, but not
swell. There is a significant tidal surge in some regions that is more like a current or continuous push, but mostly divers are talking about a back and forth motion from a few feet to many meters, depending on the size of the
swell.
For a surfer, an
ocean swell turns into a
wave when it comes into shallow water. For North America's West Coast (and Hawaii) the common origin of
big waves are the large storms that cross the Northern Pacific from Siberia to Alaska. Depending on the wind speed and size of the depression, the terms frequency, period and duration are used to describe the swell, along with swell height. This is called a groundswell, not to be confused with a sailor's local windswell, which causes waves both in deep water and near shore. A wave
crests, a swell
rolls.
As a diver, when I speak of surge it is a back and forth or in and out motion, parallel to the bottom contour and mostly perpendicular to shore. If the bottom is sloped there is some up and down to the in and out; for a gradual slope there is not much up and down. The key to surge is the reflection of the swell. The swell pushes water into the shallows (up the beach) and that water then
turns around and goes back out to sea. The swell turns into a wave if it is large enough, but surge happens whether there is a wave or not. Each ocean swell pushes water into the shore, then the
reflection swell pushes water back out to sea. (very repetitive)
Windswell typically has less duration and choppy frequency so surge from windswell is often weak and cancelled by it's brethren. Unless diving very shallow, groundswell is mostly what we feel as divers, and depending on the size of the swell I have experienced strong surge at depths up to 100'. If you are on a wall dive (or wreck hull) with a large swell hitting broadside, the surge can be mostly up and down, which can be very challenging.
Fighting surge is not conducive to good diving; work with the surge and you become one with your surroundings.
