What makes sea food have similar smell?
The American Society for Nutrition explains it thus:

The answer has to do with some interesting physiology unique to sea
creatures.Water in the open ocean is about 3% salt by weight, but the
optimal levels of dissolved minerals inside an animal cell is less
than 1%.In order to maintain fluid balance, ocean creatures must fill
their cells with amino acids and amines to counter the saltiness of
seawater. Ocean fish tend to rely on trimethylamine oxide (TMAO) for
this purpose.


Now this compound does not in itself have a particularly pungent odour, however:


The problem is that when fish are killed, bacteria and fish enzymes
convert TMAO into trimethylamine (TMA), which gives off the
characteristic “fishy” odor.This smell can be reduced in two ways.TMA
on the surface of the fish can be rinsed off with tap water.Treating
the fish with acidic ingredients such as lemon, vinegar, or tomato can
also cause TMA to bind to water and become less volatile.Thus the odor
compounds do not reach the nose.

This trimethylamine is something that is produced in small quantities by our own gut microbes, and usually is no trouble when kept under control - some however are unfortunatley prone to &quot;Fish Odour Syndrome&quot;.

I am not able to find any definitive evidence for the reason that nori smells of fish, however, an uncorroborated suggestion behind the scenes on a Wikipedia wiki page states this:


Please. Fish smells of algae, not the other way around! For the same
reason DHAs are not &quot;fish oils&quot;, but &quot;algae oils&quot;.--Rfsmit

Bolding Mine

As they are so ubiquitous in nature it would seem likely that the bacteria that convert the TMAO into TMA and make the smell would be present on the algae as well as in the fish and in our own guts.

Conclusion.

I'd claim that the bacteria and what it eats and it's waste products are to blame for the similar smell: it would seem to be the case that Japanese Nori (There mis-classified as genus Porphyra) contains amino acids and nutrients not inconsistent with the culprit bacteria growing on it (As it also like fish flesh). As I say I can not offer anything definitive.