Why E-boats posed the sharper Channel threat

Would E-boats or submarines have been more likely to disrupt the D-Day landings?

E-boats were the more serious threat because shallow water constrained submarines, while a large fast-boat attack could be very destructive.

The shallow Channel gave submarines few escape routes, while fast torpedo boats could be lethal there. Allied no-go areas, Tribal-class destroyers, and motor-torpedo-boat forces narrowed the routes an attacker could use. The same force concentration explains why Operation Neptune could draw on ships without the usual competing demands of other theatres.