Sea Movement
It's a good idea to have a basic knowledge, as getting the heading right may well mean the difference between survival and disaster.
Whereas waves arise from local winds, swells (which relate to larger bodies of water), rely on more distant and substantial disturbances. They move primarily up and down, and only give the illusion of movement, as the sea does not actually move much horizontally. This is more dominant than anything caused by the wind, so it doesn't depend on wind direction, although secondary swells may well do. It's extremely dangerous to land into wind without regard to sea conditions; the swell must be taken into consideration, although it could assume less importance if the wind is very strong.
The vast majority of swells are lower than 12-15 feet, and the swell face is the side facing you, whereas the backside is away from you. This seems to apply regardless of the direction of swell movement.

The Procedure
You will need to transmit all your MAYDAY calls and squawks (7700) while still airborne, as well as turning on your ELT, or SARBE. If time permits, warn the passengers to don their lifejackets (without inflating them, or the liferafts) and tighten seat belts, remove any headsets, stow any loose items (dentures, etc.) and pair off for mutual support, being ready to operate any emergency equipment that may be to hand (they should have been briefed on this before departure).
One passenger should be the "dinghy monitor", that is, be responsible for the liferaft. If it's dark, turn on the cabin lights and ensure everyone braces before impact (the brace position helps to reduce the flailing of limbs, etc. as you hit the water, although its primary purpose is to stop people sliding underneath the lap strap; there are different ones for forward and aft seats).
If only one swell system exists, the problem is relatively simple—even if it's a high, fast one. Unfortunately, most cases involve two or more systems running in different directions, giving the sea a confused appearance. Always land either on the top, or on the backside of a swell in a trough (after the passage of a crest) as near as possible to any shipping, meaning you neither get the water suddenly falling away from you nor get swamped with water, and help is near.
Although you should normally land parallel to the primary swell, if the wind is strong, consider landing across if it helps minimise groundspeed (although in most cases drift caused by crosswind can be ignored, being only a secondary consideration to the forces contacted on touch-down). Thus, with a big swell, you should accept more crosswind to avoid landing directly into it. The simplest way of estimating the wind is to examine the wind streaks on the water which appear as long white streaks up- and downwind. Whichever way the foam appears to be sliding backwards is the wind direction (in other words, it's the opposite of what you think), and the relative speed is determined from the activity of the streaks themselves. Shadows and whitecaps are signs of large seas, and if they're close together, the sea will be short and rough. Avoid these areas as far as possible—you only need about 500' or so to play with.