A more data-driven followup to my previous post. I tried to extract all explicit theme statements from game descriptions (i.e. statements of roughly the form "deep(er) in (noun phrase)", and I made a word cloud of the results:

Note that this doesn't necessarily mean that there are more games about the ocean than there are about caves, just that there are more games that explicitly mention going "deeper into the cave" than there are that mention "deeper in the ocean" (edit: in fact, while there are only 192 descriptions containing the word "ocean" vs. 247 descriptions containing "cave", there are 243 containing the word "dive", so I'm pretty sure that there are more ocean games than cave games overall; they just don't tend to explicitly mention the ocean.).