Negombo - is it worth spending time there or just a place to sleep before the flight?

I'm based in Negombo so I'll give you the straight answer rather than the tourism version.

What Negombo actually is: a busy coastal fishing town of around 130,000 people, with a long beach, a Dutch canal, strong Catholic colonial history (from the Portuguese and Dutch periods), and a tourist strip that developed because of the airport proximity. It is not a beautiful town in the way Galle Fort is. It is functional, genuinely local, and has real character if you engage with it rather than just sitting at a beach hotel.

The beach: honest answer - not the best beach in Sri Lanka. The sand is fine and the sea is swimmable in good conditions, but the beach gets some run-off from the town during heavy rain and it is narrower and less picturesque than the south coast beaches. People do swim and the water is warm. For lying on a sun lounger it is fine. For beach scenery it is average.

Fish market: absolutely worth the early morning visit. Negombo has one of the most active fish auction markets in Sri Lanka. The main market operates from 4am to around 7am when the overnight boats come in. It is real, loud, smells strongly of the sea, and the scale of what is landed every morning is extraordinary - tuna, swordfish, crab, lobster, and species I do not know the English names for. Go with comfortable clothes, open-toe shoes you don't mind getting wet, and a camera. Nobody will bother you.

Dutch canal: the Hamilton Canal running south from Negombo toward Colombo is a remnant of the Dutch colonial canal system and you can take short boat trips on it through the waterways. About 1-1.5 hours for a proper trip through the mangroves and fishing village sections. LKR 1,500-2,500 for a small boat. Peaceful and genuinely interesting for an afternoon.

Food: Negombo has excellent seafood restaurants because the fish literally comes off the boats nearby. Lords Restaurant and the Bijou are consistently recommended for fresh crab and prawn. For Sri Lankan rice and curry at local prices, the restaurants away from the beach strip on the main Lewis Place road are better value.

First or last night: both work. First night is logical for an afternoon arrival - 10-15 minutes from the airport, easy start to the trip, no long transfer when you are tired. Last night is useful if your flight is early morning and you want to avoid an early morning Colombo transfer. The airport is closer from Negombo than from central Colombo.