Dobbiamo controllare il nodo, altrimenti non possiamo ormeggiare.

Dobbiamo is the present tense of dovere (to have to / must) for noi (we).

So:

• io devo = I must / I have to
• tu devi = you must
• lui/lei deve = he/she must
• noi dobbiamo = we must / we have to
• voi dovete = you all must
• loro devono = they must

In this sentence, dobbiamo controllare means we have to check or we must check.

Because after a modal verb like dovere, potere, or volere, Italian normally uses the infinitive of the main verb.

So:

• dobbiamo controllare = we must check
• possiamo ormeggiare = we can moor
• voglio partire = I want to leave

This works much like English:

• we must check
• not we must we check

So controllare stays in the infinitive because it depends on dobbiamo.

Il nodo literally means the knot or the node, depending on context. Since you said the meaning is already known to the learner, the main grammar point is the article.

Italian often uses the definite article more naturally than English does. So il nodo is perfectly normal even where English might sometimes say just the knot or even omit an article in a different phrasing.

Here:

• il = masculine singular the
• nodo = knot / node

So il nodo is simply the knot.

Altrimenti means otherwise.

It introduces the consequence of not doing the first action:

• Dobbiamo controllare il nodo, altrimenti non possiamo ormeggiare.
• We have to check the knot, otherwise we can’t moor.

It is very commonly placed between two clauses like this, often after a comma. It can also move around somewhat depending on style, but this position is very natural.

Examples:

• Sbrigati, altrimenti arriviamo tardi.
  Hurry up, otherwise we’ll arrive late.
• Devi studiare, altrimenti non passi l’esame.
  You have to study, otherwise you won’t pass the exam.

These two mean different things.

• non possiamo ormeggiare = we cannot moor
• possiamo non ormeggiare = we can choose not to moor / it’s possible for us not to moor

In your sentence, the meaning is clearly we are unable to moor, so Italian uses non possiamo ormeggiare.

This is a very important distinction:

• non posso venire = I can’t come
• posso non venire = I can choose not to come / I may not come

Ormeggiare is a regular -are verb. It means to moor, to dock, or to tie up a boat.

Basic present tense:

• io ormeggio
• tu ormeggi
• lui/lei ormeggia
• noi ormeggiamo
• voi ormeggiate
• loro ormeggiano

In the sentence, it appears in the infinitive because it follows possiamo:

• possiamo ormeggiare = we can moor

It is very natural and usually preferred here because the sentence contains two linked clauses:

• Dobbiamo controllare il nodo
• altrimenti non possiamo ormeggiare

The comma helps show the pause and the logical relationship. In everyday writing, you will very often see altrimenti introduced with a comma in this kind of structure.

Not directly in the same structure.

• Dobbiamo controllare il nodo = We have to check the knot
• Abbiamo bisogno di controllare il nodo = We need to check the knot

Both can work in meaning, but the grammar changes:

• dovere + infinitive
• avere bisogno di + infinitive

So you can say:

• Dobbiamo controllare il nodo
• Abbiamo bisogno di controllare il nodo

But not abbiamo bisogno controllare.

Yes. Controllare can mean several related things depending on context:

• to check
• to inspect
• to verify
• sometimes to control, though English learners should be careful with this one

In many cases, controllare is best understood as to check rather than to control.

Examples:

• Controlla i documenti. = Check the documents.
• Devo controllare se è tutto a posto. = I have to check if everything is okay.

So in your sentence, controllare il nodo most naturally means check the knot.

Yes, you can say Noi dobbiamo controllare il nodo, but Italian usually omits subject pronouns unless they are needed for emphasis or contrast.

So the normal version is:

• Dobbiamo controllare il nodo

You might add noi if you want to stress we specifically:

• Noi dobbiamo controllare il nodo, non loro.
  We have to check the knot, not them.

Because the verb ending -iamo already shows the subject is noi, the pronoun is often unnecessary.

It is neutral and completely standard. Nothing in it is especially formal or especially casual.

• dobbiamo
• controllare
• altrimenti
• non possiamo
• ormeggiare

These are all normal standard Italian words and structures. The sentence would fit spoken or written Italian, especially in a practical or nautical context.