I have eaten expensive, mediocre fish in a great many seaside restaurants with a view of the boats that supposedly caught it. Proximity to the sea is close to useless as an indicator, and the actual indicators are learnable.
Ask what is local and what is not
The first question and it sorts most places immediately.
A restaurant that knows the answer will tell you specifically — this is from here, this comes frozen, this is farmed, this is landed at the port up the coast.
A restaurant that says everything is fresh and local is either lying or does not know, and both are informative.
The reality in most coastal tourist towns is that a proportion of the menu is imported and frozen, because the local catch is variable and a menu cannot be. That is entirely reasonable and the honest places say so.
A short menu is a strong signal
The most reliable thing on this list.
Fish is highly perishable, the catch varies by day and season, and no small kitchen can hold twenty species fresh.
A restaurant offering four or five fish, changing regularly, is buying what is available. A restaurant offering twenty, the same twenty every day, year round, is buying from a freezer.
The best version is a board rather than a printed menu, changed daily, occasionally with things crossed off as they run out.
Whole fish over fillets
Both a quality signal and a practical preference.
A whole fish can be assessed. Clear eyes, red gills, firm flesh, a clean smell. A fillet has been separated from all the evidence.
Restaurants confident in their fish will show it to you before cooking, and in many places this is standard practice rather than something you have to request.
A whole fish also cooks better, staying moister because the bones and skin protect the flesh, and it is generally cheaper per portion because there is less processing.
The barrier is that many people are uncomfortable dealing with bones, which is a skill worth acquiring for exactly this reason.
The species you have never heard of
The best value and generally the best eating.
Every coastline has fish that are excellent, abundant, cheap, and unknown outside their region because they do not export well or do not have marketable names.
These are what local people eat and what a good restaurant will be pleased to be asked about.
The internationally famous species, by contrast, are frequently overfished, expensive, and often imported even where they are native, because the local catch goes to export.
Asking what the fishermen eat is a slightly obvious question that gets an honest and useful answer more often than not.
Where the restaurant is
Directly on the harbour front with a view is the most expensive real estate and the most transient customer base. Neither of those pushes toward quality.
Two streets back, with no view, is where the local restaurants are, and the difference in both price and quality is frequently large.
The other place to look is the fishing port rather than the tourist harbour, which in many towns are separate. Whatever is near where the boats actually land tends to be serious.
Seasonality is real and specific
Fish have seasons, and eating them out of season means eating frozen, imported, or fish caught at a poor time.
Local seasons are well known to anybody in the trade and almost never published for visitors.
Asking what is in season now is another question that immediately distinguishes a place that knows from a place that does not.
There is also a sustainability dimension. Some species are under real pressure and several countries publish guidance about what to avoid. Checking that before a trip takes ten minutes and it is the sort of thing that only matters if people actually do it.
The one that surprised me
Frozen at sea is frequently better than fresh at the market.
Fish frozen within hours of being caught, properly, at very low temperature, is in better condition than fish that has been on ice for three days travelling to a market.
Which means the word frozen is not automatically a downgrade, and the honest restaurants that tell you something is frozen at sea are frequently serving better fish than the ones claiming everything is fresh.
I stopped treating frozen as a negative once I understood this, and it opened up a lot of places I would previously have dismissed.
Buying it yourself
The version I enjoy most, where the accommodation allows it.
Buying whole fish at a market and cooking it simply is cheaper than any restaurant, and it is the best way to learn what the local species actually are.
Fishmongers will gut and scale for you, and will usually tell you how they would cook it if asked, which is the most useful cooking advice available anywhere.
Grilled whole over heat with salt and lemon is genuinely difficult to improve on and requires almost nothing in the way of equipment.