Chuck,
The simple answer is that it's not simple. Unless someone has spent considerable time on the water and over the water and is familiar not only with water behavior in generaly, but local behavior, it's a crap shoot. When one is in the position of making an unscheduled, forced landing as the result of an engine failure or other condition, with blood pressure high, and concern at a peak, judgement is made even more difficult.
For those who fly over water frequently, I strongly recommend at a minimum an introduction to seaplane flying. Unfortunately, this is nearly always done from a lake or a sheltered area, which still takes a lot of the judgement out of the learning process.
I think the best that can be said is that it's an emergency, and the person forced into this situation can only make the best of it as able.
With swells in particular, the overall wave pattern isn't so important as is blending with the swell (unless one is landing against them/or waves, and into the wind). What the rest of the ocean is doing at the time isn't really relevant, compared to the position one touches down on with the swell.
What can happen, however, is an attempt to land on the crest of a swell meets an overshoot or undershoot. Where the water was just below you before, now it's just air, and for the pilot who attempts to grease the airplane onto the water, now there's a ten foot drop or more, with another swell or wave coming over the top.
An emergency water landing isn't so much about finesse as getting down and getting clear.
Or even more dangerous is a swell running with a glassy or near glassy water condition...how do you determine if they are there and the direction the swells are moving?
As addressed before, glassy water landings are in a class by themselves. Highly desceptive, depth perception is nearly impossible. An approach to landing should be made at minimum sink speed, constant descent until water contact. With angled light there's a greater chance for shadowing, but if it's truly glassy water, then having an idea of the surface winds and planning accordingly is the best bet.