I wouldn't say conditions on land are poorer... just more unstable, less predictable over a span of a million years. And an unstable environment appears to be one of the factors in the evolution of human intelligence. ie the environment changes faster than evolution can adapt the body... so one solution is to adapt the animal's behavior which requires intelligence.
That said, dolphins too are plenty intelligent. Octopus also are quite good puzzle solvers.. but are short lived.
As for predator -prey ratio https://www.mcgill.ca/newsroom/channels/news/why-arent-there-more-lions-254873 Here is something to read. Predator-prey ratio appear to be govern by the rate of growth of prey items
And prey in the ocean has a faster turn around in the ocean.. so there are more predators in the ocean than on land. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4240998/.