The laboratory simulation established that the oblique wave approach and the absence of a beach expose the breakwaters made of shaped blocks at the root of seawalls to significant wave impacts, which results in their loss of stability. The mass of blocks required to maintain the stability of the structures is significantly greater than the one calculated in regulatory documents. This can be explained by the interference of direct and reflected waves. For a wave fully reflected from a seawall, the standing wave affecting the breakwaters will reach a height twice as great as the original. For a partially reflected wave, the height of the interfered wave depends on the angle of wave approach and the design of the seawall. The paper presents a theoretical rationale and a calculation methodology and introduces the results of numerical experiments.