From what i've seen from the two of them, the HD offers much more detail in close quarters (i.e. coming into estuaries or ports) in deep fog
: there is some benefit for distiguishing rain patterns too. However, IMHO for the majority of coastal work i doubt you'd feel the 50% extra outlay in the performance differential - although i'm sure that someone will step in and correct me.
Greg's post tells you all you need to know about the lower-cost SD version. That said, i'm sure everyone would buy the HD if they were the same price!