Matthew Poole and continuators on Ecclesiastes 1:7

7 All the rivers run into the sea, yet the sea is not full : unto the place from whence the rivers come , thither they return again.

To wit, to the brink, or so as to overflow the Earth, which might be expected from such vast accessions to it. Whereby also he intimates the emptiness and dissatisfaction of mens minds, notwithstanding all the abundance of Creature comforts.

Either 1. unto the Sea, from whence they are supposed to return into their proper Channels, and then, as it is expressed, thither (i. e. into the Sea) they return again. Or 2. unto their Springs or Fountains, to which the Waters return by secret passages of the Earth, as is manifest from the Caspian Sea, and reasonably supposed in other places. Or rather 3. unto the Earth in general, from whence they come or flow into the Sea, and to which they return again by the reflux of the Sea. For he seems to speak of the visible and constant motion of the Waters, both to the Sea and from it, and then to it again in a perpetual reciprocation: which agrees best with the former similitudes, v. 5, 6.

Marginal references: Job 38. 10. Psa. 104 8. 9.; Heb. unto the place that the Rivers go, thither they return to go.