The cruises compared here
Hop-on hop-off sounds like a network of stops. On the Tagus it is two: Terreiro do Paço in the centre and the Belém river terminal. The Yellow Boat ticket lasts 24 hours and lets you use the boat as often as you like between them.
With three sailings a day from each end, what you are really buying is a boat ride to Belém and a boat ride back. Seen that way, it has a direct competitor that is cheaper and better reviewed.
1What the ticket covers
The listing sells unlimited travel on the sightseeing boat for 24 hours, with recorded commentary. The operator, which sells it under the Yellow Boat name, gives the commentary languages as Portuguese, English, French and Spanish. The listing describes a ride of about 1.5 hours towards Belém, with a choice of sitting inside or out on deck.
The operator's own page also mentions entry to the Carris Museum and free travel on Carris trams with its ticket. The Civitatis listing does not include either, so do not count on them when booking there.
2The timetable is the catch
On Mondays, Wednesdays, Fridays and Saturdays the boat leaves Terreiro do Paço at 11 am, 2 pm and 4 pm and leaves Belém at 12 pm, 3 pm and 5 pm. Miss the 5 pm from Belém and you are coming back by tram.
On Tuesdays, Thursdays and Sundays the listing shows departures from Terreiro do Paço only, at 11:30 am, 1:30 pm, 3:30 pm and 5:30 pm, and states that recorded commentary is not available. The operator is more direct: those sailings are run by a partner, with no stop at Belém and no audio guide. On those days there is nothing to hop on or off.
3The cheaper boat that also stops in Belém
The FRS Portugal boat tour leaves from the same river station, costs from €19.95 and has an option called boat tour disembarking in Belém. The listing gives departures for it from Terreiro do Paço at 10:30 am, 2:30 pm and 4:30 pm, and from Belém at 11 am, 3 pm and 5 pm.
It has 765 reviews and a score of 7.8, against 82 reviews and 7.4 for the Yellow Boat. It is tagged wheelchair accessible and pet-friendly; the Yellow Boat listing says it is not wheelchair accessible. The one thing the listing does not spell out is where in Belém the boat docks, so look for that on your voucher.
4When the Yellow Boat still wins
Commentary in a language you can follow is its strongest point. Four languages are confirmed by the operator, while the FRS listing is tagged English only and its operator does not list the audio guide languages. If you want Spanish or French narration on a river trip to Belém, the Yellow Boat on one of its four full days is the safer booking.
The 24-hour validity also helps if you start in the afternoon. A 4 pm boat out one day and a midday boat back the next are both covered by one ticket, provided both days are ones with a Belém stop.