This does not quite match the details in the post (in particular, there is no ‘time loop’), but could this be Twenty Trillion Leagues Under the Sea (2014) by Adam Roberts? In this fantasia on Jules Verne’s Twenty Thousand Leagues Under the Sea, the French submarine Plongeur suffers a malfunction:
The vanes on the exterior of the vessel tilted, and more air was pumped from the main tanks; at the same time, the four smaller orientational tanks shifted ballast between them. The Plongeur tilted forward and resumed its descent.
Almost at once, disaster struck.
There was a cacophonous report, like a cannon firing. The whole vessel shook and trembled. An alarm sounded a flute-like, almost musical noise. The angle of the deck did not increase, but the boat began to fall much faster. Nobody on the bridge needed to look at the suddenly spinning numbers of the depth gauge—everybody registered the suddenly accelerating descent in their guts.
The depth gauge spins deeper and deeper, past one hundred thousand metres, far beyond the deepest oceanic trench. The Plongeur should have been crushed by the pressure, but somehow it survives.
‘So, we must address the possibility that ours is … is no longer a terrestrial location. […] I do not preten to be able to explain how the transition occurred—or where we now find ourselves. But I am emboldened to go further in my speculations. […] I suggest we have slipped from our material dimension into another. […] Imagine that we have passed through some … portal. Imagine that we have moved from the finite Atlantic into an infinite ocean. A body of water literally without limit.’
The novel goes to describe the wonders and horrors of this infinite ocean, and the emotional and intellectual response of the crew.
(You can see some of the author’s early thoughts on this novel on his blog.)