Learn Spanish (Latin America): Él siguió remando a lo largo del túnel oscuro.

The phrase for "along" is a lo largo de. Because the following word is the masculine article el (for el túnel), the preposition de and the article el must combine to make the contraction del.

Why use siguió remando to say "kept rowing"?

To say you "keep doing" or "continue doing" an action in Spanish, you use the verb seguir followed by the gerund (the -ando or -iendo form of the verb). Here, siguió (he kept) pairs with remando (rowing) from the verb remar.

Could I use a través del túnel instead of a lo largo del túnel here?

You could, but it changes the mental picture slightly. A través de means passing "through" something, while a lo largo de emphasizes traveling "along the length" of it. Since a tunnel is a long, extended space, a lo largo de paints a great picture of moving down its entire length.

Why is there an accent mark on túnel?

Words ending in a consonant other than n or s naturally place the spoken stress on the very last syllable. Since the stress in túnel falls on the second-to-last syllable (TÚ-nel), it needs a written accent mark to show that it breaks the normal pronunciation rule.