Water Monster reports in District of Columbia
2 cases, best-documented first. Page 1 of 1.
We map claims, not creatures. A case is what some source says someone saw or heard, with the link to that source. It is not proof that anything was there, and a retelling counts once, not as a second sighting.

Water Monster · Washington, District of Columbia · 1947
Fishermen and an aquarium owner claim a sea monster was sighted in Puget Sound waters, prompting a $5,000 reward for its capture.

Water Monster · Washington, District of Columbia · 1896
Fishermen in Tacoma claim to have captured two sea serpents, one of which was killed and the other a female that escaped.