Ive noticed that when engineer dies on water, whatever killed it wont retarget anything untill it hits the bottom. Deeper the sea is, longer it takes for it to sink and more dps is wasted. Its quite easy to replicate this bug, heres a quick test i just did:
http://faforever.com/faf/vault/replay_v ... id=1498982
Every 4th or 5th engi on average does that in that replay. Ill try on other hover and navy units.