1. canal: a passage of water *that* boats can travel canal: a passage of water *that* boats can travel

[What is the part of speech of *that* in the phrase?]

2. Boats can travel a passage of water.
3. Boats can travel on a passage of water.
4. Boats can travel through a passage of water.
[Which one is grammatical of the three?]
..................
Do you mean #4 is right?

canal: a passage of water that boats can travel
a passage of water through which boats can travel
a passage of water which boats can travel through
a passage of water *where* boats can travel
a passage of water *that *boats can travel

Travel is an intransitive verb, so 'that' is a relative adverb. We can use 'that' instead of 'where'. 'Where' is a relative adverb in this phrase,isn't it?

2 answers

1. canal: a passage of water *that* boats can travel canal: a passage of water *that* boats can travel
[What is the part of speech of *that* in the phrase?]
Already answered in earlier post.
2. Boats can travel a passage of water. no
3. Boats can travel on a passage of water. no
4. Boats can travel through a passage of water. yes
[Which one is grammatical of the three?]
..................
Do you mean #4 is right? yes

canal: a passage of water that boats can travel
a passage of water through which boats can travel yes
a passage of water which boats can travel through yes
a passage of water *where* boats can travel yes
a passage of water *that *boats can travel no

Travel is an intransitive verb, so 'that' is a relative adverb. We can use 'that' instead of 'where'. 'Where' is a relative adverb in this phrase,isn't it?
Yes. Study this:
https://english.tutorvista.com/grammar/relative-adverbs.html
Which phrase from the passage refers to a key element found in myths?