This is the 5481st most frequent Russian word.
подводной
"Подводной" means "underwater" in English.
Here "подводной" refers to something 'underwater', describing the submarine.
Подводной лодкой управляет опытный капитан.
The submarine is operated by an experienced captain.
"Подводной" here is an adjective describing the 'world' as being beneath the water.
Мы исследуем подводной мир океана.
We are exploring the underwater world of the ocean.
In this instance, "подводной" is used figuratively to mean 'hidden', referring to an unseen difficulty.