Why does lemon juice reduce the "fish" odor of sea food — specifically fish?
There are 2 cases, both related to the acid-base reactions. Both are also partial reasons why so many fish recipes use lemon juice.
Fish, especially sea fish, naturally contain trimethylamine-N-oxide $\ce{(CH3)3N-O}$ that, after death, gets enzymatically reduced to trimethylamine $\ce{(CH3)3N}$, the source of ammonia-like fish odour.

Trimethylamine N-oxide is an osmolyte found in saltwater fishes, sharks, rays, molluscs, and crustaceans. It is considered as a protein stabilizer that may serve to counteract urea, the major osmolyte of sharks, skates and rays. It is also higher in deep-sea fishes and crustaceans, where it may counteract the protein-destabilizing effects of pressure.TMAO decomposes to trimethylamine (TMA), which is the main odorant that is characteristic of degrading seafood.
TMAO is biosynthesized from trimethylamine, which is derived from choline.

In acidic environment, TMA forms water soluble, non volatile, odourless trimethylammonium salts, what eliminates the volatile smelly trimethylamine.
$$\ce{(CH3)3N + H+ &lt;=&gt;&gt; (CH3)3NH+ }$$
It is the direct analogy to the smelly volatile ammonia and odourless ammonium:
$$\ce{NH3 + H+ &lt;=&gt;&gt; NH4+}$$

Another source of fishy smell/taste, especially of some sweet water species like carp or catfish, is geosmin, a polycyclic alcohol, being a derivate of decahydronaphthalene(decaline). It is released after death of some Streptomyces or cyanobacteria cells which gives a muddy or earthy taste, and the human smell sensors are extremely sensitive to it (5 ppt).
Geosmin is also responsible for smell of the soil after raining, of some beet roots and even of drinking water of surface origin. There is speculation the nose sensitivity for geosmin is a evolutionary response for the need of efficient searching for water.
Geosmin reportedly breaks down in acidic environments.