I think marathon swimming makes you work more on your upper body, your abdomen, trapezius, and your arms. When you finish a race, your arms and abdomen are more tired than your legs. Your legs are tired because you've just sprinted for the line, but at the end of the race, when you start to relax, it's your arms that hurt.
Pool swimmers are very slim, and they're stronger. Marathon swimmers have to train an awful lot, and they end up losing a bit of muscle mass. So we need to have a bit more fat, though that's not the case with me. Many swimmers have a little bit of extra fat though, so that they can deal with the cold and get through what is an exhausting event.
The difference between the swimming marathon and the pool events is mainly down to the weather. Nature always gives us external factors to deal with.
With marathon swimming, I think you have to use your head more than you do in pool events, which are more automatic. You can just go out and replicate your training pace in competition. With the marathon, though, you have to contend with nature, physical contact, and time.
