I was looking through an old collection of riddles that I had collected to use in an upcoming session of Dungeons and Dragons. Came across one where I had misplaced the answer! Now I'm going crazy with trying to figure out the answer.

"The measure of our depths is not an easy thing to tell.
We live below the ocean waves and we like it well.
Though some may claim to understand just how to find our home, trust them not unless they use us when they roam.
We bridge the sea and the land.
Speak now and understand our final clarion call."

I remember that my players thought it was Currents like water currents, but that wasn't answer. I think it might have had to do with maritime navigation etc. Help me out before I lose my mind!