Does “onto” suggest or imply that the subject is coming from above?

The raging waves crashed onto the hull of the derelict ship.

I was told that "onto" suggests that the waves are coming from above, so "against" would be a better choice. Is this true? And also since it "suggests" it doesn't mean it "implies" and therefore it is wrong to use "onto"?

To some extent, but not necessarily

Usually, the positional relationship between two objects is understood because we know how the physics work. We know, for example, that waves crash down because of gravity, so the wave had to be higher than the boat at some point because we understand this basic aspect of physics. Onto works in your example because of this.

However, one could just as easily talk about a sticker stuck onto the bottom of a bottle or a child latching onto its parent's leg, etc. In these examples, onto is more accurately understood as "covering in some way" or being "against the outer surface."

Onto does not specify that the bottle was lifted and tilted so that the sticker could be applied or that the child ran laterally towards the parent before latching onto the leg. We understand that these actions had to occur because that is the normal way the child or sticker would be onto the bottle or leg.

• I think I'd be inclined to say to some extent rather than not really. If I thought the horizontal force of the waves (as opposed to the displaced upwards, then falling down onto the deck force) was more relevant, I might be more likely to use into. – FumbleFingers Jan 30 '19 at 19:33
• +1. The chihuahua made a mighty leap up onto the sofa. And then it jumped back down onto the floor. Then it went out onto the fire escape and jumped across the alleyway onto the fire escape of the adjacent building. – Tᴚoɯɐuo Jan 30 '19 at 20:49
• @fum I think that's a fair point. I've updated the title of the answer to reflect this. – Rykara Jan 30 '19 at 22:53
• rpeinhardt: It's a good job your comment "pinged" me. I'd intended to upvote the answer anyway, but apparently forgot to do so after posting my comment. So my upvote now isn't so much to recognise your edit - more a shamefaced acknowledgement that I sometimes have "senior moments" and forget what I'm doing in the middle of some complex task (such as commenting and upvoting, probably at the same time as making a cup of tea! :) – FumbleFingers Jan 31 '19 at 13:16

You're correct, "onto" usually suggests a meaning like "from above", but it could be used flexibly.

In this case, also, we're talking about raging waves (which I assume to be at least a little tall) and a broken ship (which I assume does not tower above the water too much), so "onto" would work well here.