Harwich took the lead in the 15th minute with a well-taken goal when, from 30 yards, Billy Buckle (pictured) accurately lobbed the ball over the goalkeeper and into the net.
Just past the midway stage of the first half, the Debenham goalkeeper advanced outside his penalty area and handled Ewan Livingston's goalbound shot to receive and an inevitable red card.
On the stroke of half time, Harwich scored their second goal when a dangerous high cross was headed into his own net by a defender.
In the second half, Buckle broke through to calmly net his second and increase the Shrimpers' lead to 3-0.
Ten-man Debenham fought bravely but Harwich were passing the ball well to use their advantage to good effect and Buckle completed his hat-trick with a looping header.
Charlie Carr came within a whisker of scoring for Harwich when his shot rocketed against the angle of the post and crossbar.
Harwich brought on two teenage strikers who used the ball well and went on to score. The fifth goal arrived when Logan Stokes played through Ethan Kirk (son of former Harwich striker, Wayne), who netted with crisp, low drive. Stokes made it six for Harwich when he neatly fired home from Livingston's cross.
Harwich looked strong all-round and manager Luke Harrison was happy with his team's promising start.

Harwich team: Jack Lines, Josh Hoes, David Timperley, Oakley McNair, Reece Harte, Hayy Kabangu, Ewan Livingston, Elliot Johnson, Charlie Carr, Cody Harrison and Billy Buckle. Substitutes: Cameron Ikeda, Logan Stokes, Ethan Kirk and Ahmed Turay (all used).