Se la maro restos tiel trankvila, tia vojaĝo per boato estos tre amuza.

Se means if. It introduces a condition:

• Se la maro restos tiel trankvila... = If the sea remains that calm...

This is a very common way to build conditional sentences in Esperanto.

Restos is the future tense of resti (to remain, stay).

• resti = to remain
• restas = remains / is remaining
• restos = will remain

So la maro restos tiel trankvila means the sea will remain so calm or if the sea stays that calm.

Esperanto often uses the future tense in both parts of a future-looking if sentence, unlike English, which often says If the sea stays calm, the trip will be fun.

That is a normal Esperanto pattern. Esperanto does not avoid the future tense after se in the way English often does.

So this is natural Esperanto:

• Se li venos, mi iros. = If he comes, I will go.
• Se pluvos, ni restos hejme. = If it rains, we will stay home.

In English, we usually use the present tense after if, but Esperanto can use the tense that matches the actual time being talked about.

They are related, but they do different jobs.

• tiel = so, in such a way, to that degree
• tia = such a, that kind of

In the sentence:

• tiel trankvila = so calm / that calm
• tia vojaĝo = such a trip / that kind of trip

So:

• tiel modifies an adjective or manner/degree word
• tia is an adjective describing a noun

A useful comparison:

• La maro estas tiel trankvila. = The sea is so calm.
• Tia vetero plaĉas al mi. = Such weather pleases me.

Because trankvila describes the noun maro.

• la maro = the sea
• trankvila = calm

So it is an adjective, and adjectives in Esperanto usually end in -a.

If you used trankvile, that would be an adverb meaning calmly, which would describe an action, not the sea itself.

Compare:

• La maro estas trankvila. = The sea is calm.
• Li parolas trankvile. = He speaks calmly.

Tia adds the idea of such a or that kind of. It connects the second part of the sentence to the first.

The idea is:

• If the sea stays that calm,
• then a trip like that / such a boat trip will be very enjoyable.

Without tia, the sentence would still make sense, but it would lose some of that connection:

• ...vojaĝo per boato estos tre amuza. = ...a trip by boat will be very fun.
• ...tia vojaĝo per boato estos tre amuza. = ...such a boat trip will be very fun.

Per usually means by means of, using.

So:

• vojaĝo per boato = a trip by boat
• literally, a trip by means of a boat

This is a very common Esperanto way to express means of transport:

• per aŭto = by car
• per trajno = by train
• per aviadilo = by plane

You could also say boata vojaĝo (a boat trip), and that would be natural too. But vojaĝo per boato emphasizes the means of travel very clearly.

Because there is no direct object here.

The sentence is built around forms of esti and resti, plus descriptions:

• la maro restos tiel trankvila
• tia vojaĝo per boato estos tre amuza

In both parts, the nouns are being described, not acted upon as direct objects.

You use -n mainly for direct objects and sometimes for motion toward something. Neither of those applies here.

Tre means very. It intensifies an adjective or adverb.

• amuza = fun, amusing
• tre amuza = very fun / very enjoyable

It works much like English very:

• tre granda = very big
• tre rapide = very quickly

It can cover several nearby ideas, depending on context.

• amuza often means amusing
• in many contexts it can also mean fun or enjoyable

In this sentence, tre amuza is best understood as very enjoyable or great fun, not necessarily humorous.

So the sense is that the boat trip would be pleasant and enjoyable because the sea is calm.

Yes, Esperanto word order is fairly flexible, though some orders sound more neutral than others.

The given sentence is very natural and straightforward:

• Se la maro restos tiel trankvila, tia vojaĝo per boato estos tre amuza.

You could rearrange parts for emphasis, for example:

• Tia vojaĝo per boato estos tre amuza, se la maro restos tiel trankvila.

That still means the same thing. However, the original order is probably the clearest for a learner.

Yes, it is natural here because la maro refers to the sea as a known or specific thing.

In many contexts, broad natural things often take la in Esperanto when treated as a definite thing:

• la suno = the sun
• la ĉielo = the sky
• la maro = the sea

If you said just maro, it would sound more like a sea or sea in a more indefinite sense.

A fairly literal version would be:

If the sea will remain so calm, such a trip by boat will be very enjoyable.

That is closer to the Esperanto structure than the most natural English translation. A more natural English version would usually be:

If the sea stays this calm, a boat trip like that will be great fun.

Both reflect the same Esperanto sentence, but the first one helps show how the grammar is built.