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Let's consider the following sentence, which I just found in Google as an illustrative example:

Наиболее опасные из акул – тигровые акулы, которые обитают в тропических водах.

Two interpretations come to my mind:

(1) The most dangerous sharks are the tiger sharks, which live in tropical waters.

(2) The most dangerous sharks are the tiger sharks that live in tropical waters.

Sentence (1) contains a non-restrictive clause and says two separate things, the first one being that the most dangerous sharks are the tiger sharks, and the second one being that the tiger sharks live in tropical waters. Sentence (2) contains a restrictive clause and says only one thing, namely, that the most dangerous sharks are those tiger sharks that live in tropical waters. Sentence (2) implies that there are also tiger sharks that live in other waters and that those tiger sharks are less dangerous.

My question is this: Are clauses with который restrictive or non-restrictive by default - that is, in situations where the meaning of который is unclear from the context? In other words, which meaning should be assumed when the context allows both?

You can't see the difference between a restrictive and non-restrictive который, but you can hear it.

If the noun phrase that который refers to is stressed and you can sort of "hear" the comma, i.e. the overall tone falls off and starts picking up again on который, it's non-restrictive.

If it's restrictive, the noun phrase and который have about the same emphasis (or lack thereof), and there's no audible break or "reset" of the intonation, just as if the comma wasn't there.

Your particular example, intuitively, feels non-restrictive. It could be restrictive, one would need to hear it to be sure. Just an inference from the meaning; real-world facts suggest a shark is more likely to be deemed the most dangerous based on its species alone, rather than species plus habitat. However, if it was species plus habitat, I feel I'd expect it to be expressed more clearly — say, те из тигровых акул, что обитают, etc.

It all depends on context; the rules are not set in stone; it is neither invariably/intrinsically non-restrictive nor invariably/intrinsically restrictive by default. I've come up with two colloquial sentences to illustrate the difference:

Если и есть что-то хуже нарциссов, так это люди, которые не умеют держать себя в руках.

• ... people who ... rather than ... people, who ...

In English, this can only reasonably be interpreted as a (2) restrictive relative clause (without a preceding comma); if you see it as a (1) non-restrictive relative clause (with a comma), it sounds as if all the people in the world act in the same unpleasant manner. Which would be an illogical thing to say.

И ради этого ты готов пожертвовать своими товарищами, которые помогали тебе все это время?!

• ... colleagues, who ... rather than ... colleagues who ...

In English, this one, on the other hand, can only reasonably be interpreted as a (1) non-restrictive relative clause (with a preceding comma); if you see it as a (2) restrictive relative clause (without a comma), it sounds as if there are some colleagues you will NOT sacrifice: those who have never helped you before. Which would be an illogical thing to say.

Some languages (English, French etc) make a distinction between a non-restrictive relative clause and a restrictive relative clause with a preceding comma, while others (Russian, German etc) do not, at least in writing, with an ever-present preceding comma.