What I expected to happen was...:
That Endermites would take water damage when touching water.
I thought that they would take water damage like Endermen do.^{But they will drown so that works}
What actually happened was...:
It just walked about the bottom of the lake and eventually suffocated. ^{And I learned that Endermites and Silverfish can't swim (bug?)}
Steps to Reproduce:
1.Erect a river (or a lake.. whatever...)
2.Spawn an Endermite in to the water (or spam enderpearls in to a lake)
3.Observe