For every asteroid or meteorite that hits dry land, two hit the sea. If a Manicouagan-sized asteroid struck the deep ocean (but not above a trench),how would the H2O vapour, chlorine, iodine bromine and other elements affect the climate, and for how long? Also, if it had thousands of feet of water to penetrate, would it leave a large crater on the sea bed? For sure it would create massive tsunamis, so is there any evidence for these in the geological record? The Manicouagan crater is far from being the largest on Earth, but is only slightly smaller than the Chicxulub example. It was caused by an asteroid 5km in diameter traveling at 25km per second, though friction with the atmosphere probably reduced this slightly before it struck the ground.
Manicouagan, however, is just an example, we are talking here about a hypothetical asteroid of this size striking the deep ocean. Don't be misled by the word hypothetical; given the number of large impacts which have happened on land there must have been some which landed in deep ocean. How deep? Let's call it 6,000 feet, which is nowhere near a record depth.