The cruises compared here
On the Tagus the sunset premium is real. Both operators that sell a daytime cruise also sell a sunset one, and in both cases it costs roughly €15 to €25 more per person for the same water and much the same route.
So the question is what that extra money buys in each case, and whether a daytime slot would give you nearly the same trip for about half the price.
1The two sunset cruises side by side
The sailboat sunset cruise is run by Taguscruises from Doca do Bom Sucesso in Belém. It lasts two hours, sails from Belém towards Alfama and back under the 25 de Abril Bridge, and uses a Jeanneau Sun Odyssey yacht with 10 or 11 places. One beer or soft drink is included. It starts from €45.
The sunset cruise from Terreiro do Paço is run by FRS Portugal on one of its large sightseeing boats. It lasts 80 to 90 minutes, heads west past Praça do Comércio and under the bridge to the Belém Tower, then returns. One welcome drink is included, and here wine is among the choices. It starts from €34.95.
2What the premium buys on each
On the big boat the daytime tour costs from €19.95 and the sunset one from €34.95. For the €15 difference the listing adds a welcome drink and the timing. The route is the same and the trip is not longer. The operator's own page also mentions light snacks on its sunset cruise, but the listing does not, so do not book on that basis.
On the sailboat the daytime ride starts from €20 and the sunset one from €45. Here the gap is wider, but the daytime ride is sold as a one-hour slot at 11 am or a two-hour slot at 3 pm, and the listing does not say which of them the starting price refers to. The sunset sail is always two hours, in a group capped at 10 or 11 instead of 14.
3Why the sailboat is the one to pay for
The scores point the same way as the arithmetic. The sunset sail has 9.4 from 1,035 reviews; the big sunset boat has 8.4 from 526. A difference of a full point across that many reviews is not noise.
The practical gap is group size. A yacht with 11 places and a ship with a cafeteria are different evenings, even if the sky is the same. If you are paying a premium for atmosphere, pay it where the boat adds to it.
4When the daytime cruise is the better buy
Take a daytime cruise if you care more about seeing the monuments than the light. The Belém Tower, the Padrão dos Descobrimentos and the bridge are on every one of these routes, and a slot at 11 am or 3 pm shows them in full daylight.
Daytime also suits anyone with a dinner booking or tired children. The sunset sail leaves two hours before sunset according to the operator, so it ends at dusk in Belém with the trip back to the centre still ahead of you.
And if the forecast is cloudy, a sunset cruise is just a more expensive cruise. Both can be cancelled free up to 24 hours before, so check the forecast the day before and move to a daytime slot if it looks grey.
5Small print that catches people out
On the sailboat, the included drink is a beer or a soft drink. You may not bring red wine or beer in glass bottles, and the listing sets a €50 charge, paid at check-in, for bringing and drinking your own alcohol. Plan a glass of wine for after the cruise.
On the big boat, the listing tags the cruise wheelchair accessible if you declare it when booking. On the sailboat only some areas are accessible and an accompanying person is required. Both cruises leave from different parts of town, about 7 km apart, so check your voucher before you set off.