Answer:
Crest
Step-by-step explanation:
The crest of the wave crashed over the railing of our boat.
The correct word to fill in the gap should be crest. The crest represents the highest part of a wave while the trough is the opposite, that is, the lowest part of a wave. Hence, the part of a wave that is capable of crashing over the railing of a boat is the peak or the crest.