Discovered this when building a silverfish farm in my stronghold on a 1.13 Realms server. The silverfish spawn, drop into a 1-high water stream, and immediately begin taking damage. This does not happen when they are spawned manually from spawn eggs or when released from infested blocks. It appears to be related to their "air meter", rather than their hitbox (that's another issue entirely).

I tried this again in a separate 1.13.1 single player world, as the Realms server is a version out-of-date, and the problem is still relevant.