Many mammals on land seem to be quite dangerous to humans: e.g., tigers, lions, elephants, hypos etc. However, their sea-dwelling counterparts (e.g., orcas, dolphins, sea lions, whales etc.) are relatively friendly to humans (and in contrast to say sharks). Is there a simple reason for this?

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    $\begingroup$ Is there an objective way to measure friendliness of wild animals? $\endgroup$ – kmm Mar 23 '19 at 23:46
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    $\begingroup$ There is only one sea mammal specialized in eating seal sized prey and mammals, orcas. Why only one? that's tough, it's the solution to your query. Land based mammals prey substantially on large mammals, bears, big cats, wolves... When mammals size up another animal they think: "what is that animal shape doing? is it a food or a monster?" and "what is that face?" when they look at other animals. When sea mammals sea us they think "that's an alien from another world, he swims like a clown but his face is cool" $\endgroup$ – aliential Mar 24 '19 at 8:05

More encounters on land

There are more land animals that meet humans walking around than there are sea animals that meet humans swimming around. If fewer attacks happen on humans in the sea environment, it might just be because there are fewer encounters with marine mammals than with land mammals.

Greater species diversity on land

In your list used as example, you cherry-picked a few aggressive(-ish) land mammals but the vast majority of land mammals are not aggressive toward humans. The fact that there are no marine mammal that are aggressive toward humans might just be because there are too few fewer marine mammal species.

Pseudoreplication and phylogenetic signal

I would argue that your sample size is not as large as you might think. There are only four lineages of mammals that evolve to live in the seas. There is therefore a phylogenetic signal and ignoring it would be a problem of pseudo replication.

For example, I could very well think of territorial behaviour (which is where a lot of aggressive behaviour comes from) as having a strong phylogenetic signal.

Why considering only humans?

Your post title talks about aggressiveness in general and the content of your post talks about attacks against humans. It sounds that considering only attacks against humans to be very unfair. Orcas, typically, are fierce and violent predators although no orcas have ever attacked a human in the wild.

Are they really that friendly to humans?

There has been a number of attacks in captivity (well known cases with orcas). Also, there have been a number of whales attacks against boats (according to this BBC article).

In short...

In short, I think your comparison is unfair. I think you are looking for an explanation for a pattern that does not really exist.

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    $\begingroup$ I think there are actually 4 lineages of marine mammals: cetaceans (whales & dolphins), pinnipeds (seals & walruses), sirenia (manatees and dugongs), and mustelids (sea otters). Maybe 5, if you include polar bears - which aren't all that friendly to humans. Wikipedia has a diagram: en.wikipedia.org/wiki/Marine_mammal $\endgroup$ – jamesqf Mar 24 '19 at 4:51
  • $\begingroup$ @jamesqf Thanks for correcting! I forgot sirenia and the sea otters. I edited the post $\endgroup$ – Remi.b Mar 24 '19 at 5:37
  • $\begingroup$ Another example of a predatory cetacean would be the sperm whale. But since they prey mostly on squid that live at depths of 300-800 m or more, you are not in much danger from them. Unless you are Captain Ahab, of course - and that was pure self-defense on Moby Dick's part :-) (Yes, I know that's fiction, but it was inspired by actual events, notably the sinking of the Essex: en.wikipedia.org/wiki/Essex_(whaleship) ) $\endgroup$ – jamesqf Mar 25 '19 at 18:06

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