Learn Hardcore Swahili: Mashua zinaonekana baharini asubuhi. - Boats are seen in the ocean in the morning.

zi-: subject-agreement prefix for noun class 6 (used with mashua)
na: present-tense marker (TAM)
onekana: verb root -onekana (“to be seen” or “to appear”)
Put together, zi-na-onekana literally means “they (class 6) are seen” or “they appear.”

Is kuonekana just the passive of kuona, and can I use it like “boats are being seen”?

No. Although kuonekana looks like a passive, in Swahili it’s an intransitive (stative/inchoative) verb meaning “to appear” or “to be visible.” You don’t need an agent (“by someone”). If you really want to say “to be seen by someone,” you would add na plus the agent: kuonekana na watu (“to be seen by people”).

Why does bahari become baharini? What does the -ni ending do?

In Swahili, adding -ni to many nouns turns them into locatives (“in/at/on that noun”). So:

bahari = “sea”
baharini = “at/in the sea”
You don’t need an extra preposition; the -ni suffix handles the location.

What part of speech is asubuhi, and why doesn’t it take a suffix like -ni?

Asubuhi is a noun (“morning”) being used adverbially to indicate time. As a time-word it usually stands alone without a locative suffix. If you wanted “in the morning” more explicitly, you could say asubuhi ya tukio or just rely on context: asubuhi already implies “in the morning.”

Why is the subject mashua not omitted, since the verb has a prefix?

Swahili allows you to drop noun subjects because the verb prefix (here zi-) already identifies them. However, learners and speakers often keep the noun for clarity or emphasis. Omitting it would still be grammatically correct: Zinaonekana baharini asubuhi.

Can I add a phrase like “every” morning or “early” morning in this sentence?

Yes. For “every morning,” insert kila:
Mashua zinaonekana baharini kila asubuhi.
For “early morning,” use mapema after asubuhi or as a modifier:
Mashua zinaonekana baharini asubuhi mapema.